Immunoglobulin light chain Constant domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) light chain constant (C) domain. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determine the type of immunoglobulin: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVSIFPPSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKDEYGRHNS 190
Cdd:cd07699   1 APSVTIFPPSSEELSSGKATLVCLINKFYPGFATVTWKVDGSTVSSGVTTSKTEQQS-DNTYSMSSYLTLSSSDWNKHKV 79

                        90       100
                ....*....|....*....|
gi 12002896 191 YTCEATHKTSTSPIVKSFNR 210
Cdd:cd07699  80 YTCEVTHEGLSSTITKSFNR 99

Immunoglobulin light chain Constant domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) light chain constant (C) domain. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determine the type of immunoglobulin: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVSIFPPSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKDEYGRHNS 190
Cdd:cd07699   1 APSVTIFPPSSEELSSGKATLVCLINKFYPGFATVTWKVDGSTVSSGVTTSKTEQQS-DNTYSMSSYLTLSSSDWNKHKV 79

                        90       100
                ....*....|....*....|
gi 12002896 191 YTCEATHKTSTSPIVKSFNR 210
Cdd:cd07699  80 YTCEVTHEGLSSTITKSFNR 99

Immunoglobulin C-Type;

                           10        20        30        40        50        60        70
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 12002896    129 ASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKDEYGRHNSYTCEATHKTSTSP 203
Cdd:smart00407   2 ATLVCLVSGFYPPDITVTWLRNGQEVTEGVSTTDPLKNS-DGTYFLSSYLTVPASTWESGDVYTCQVTHEGLKEP 75

Immunoglobulin V-Type;

                           10        20        30        40        50        60        70
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 12002896     19 VTIACKA--NQDINKYIAWYQHKPGKGPRLLI----HYTSTLQPGIPSRFSGSG--SGRDYSFSISNLEPEDIATYYC 88
Cdd:smart00406   2 VTLSCKFsgSTFSSYYVSWVRQPPGKGLEWLGyigsNGSSYYQESYKGRFTISKdtSKNDVSLTISNLRVEDTGTYYC 79

Immunoglobulin light chain Constant domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) light chain constant (C) domain. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determine the type of immunoglobulin: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVSIFPPSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKDEYGRHNS 190
Cdd:cd07699   1 APSVTIFPPSSEELSSGKATLVCLINKFYPGFATVTWKVDGSTVSSGVTTSKTEQQS-DNTYSMSSYLTLSSSDWNKHKV 79

                        90       100
                ....*....|....*....|
gi 12002896 191 YTCEATHKTSTSPIVKSFNR 210
Cdd:cd07699  80 YTCEVTHEGLSSTITKSFNR 99

Immunoglobulin C1-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896   114 VSIFPPSSEQLtSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVlNSWTDQDSKDSTYSMSSTLTLTKDEYGRHNSYTC 193
Cdd:pfam07654   1 VYVFPPSPEEL-GKPNTLTCLVTGFYPPDITVTWLKNGQEVTEGV-KTTPPSPNSDWTYQLSSYLTVTPSDWESGDEYTC 78

                  ....*..
gi 12002896   194 EATHKTS 200
Cdd:pfam07654  79 RVEHEGL 85

Immunoglobulin Constant-1 (C1)-set domain; The members here are composed of C1-set domains, classical Ig-like domains resembling the antibody constant domain. Members of the IgC1 family are components of immunoglobulin, T-cell receptors, CD1 cell surface glycoproteins, secretory glycoproteins A/C, and major histocompatibility complex (MHC) class I/II molecules. In immunoglobulins, each chain is composed of one variable domain (IgV) and one or more IgC domains. These names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. The IgV domain is responsible for antigen binding, while the IgC domain is involved in oligomerization and molecular interactions. The structures in C1-set are smaller than those in the V-set; they have one beta sheet that is formed by strands A, B, E, and D and the other strands by G, F, C, and C'.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 113 TVSIFPPSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTdQDSKDSTYSMSSTLTLTKDEYGRHNSYT 192
Cdd:cd00098   1 TVTLLPPSPEEKGGGKVTLVCLVSGFYPKDITVTWLKNGVPLTSGVSTSSP-VEPNDGTYSVTSSLTVPPSDWDEGATYT 79

                        90
                ....*....|....*.
gi 12002896 193 CEATHKTSTSPIVKSF 208
Cdd:cd00098  80 CVVTHESLKSPLSKTW 95

T cell receptor (TCR) beta chain constant immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the T cell receptor (TCR) beta chain constant immunoglobulin domain. TCRs mediate antigen recognition by T lymphocytes, and are composed of alpha and beta, or gamma and delta, polypeptide chains with variable (V) and constant (C) regions. This group includes the variable domain of the beta chain. Alpha/beta TCRs recognize antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. The antigen binding site is formed by the variable domains of the alpha and beta chains, located at the N-terminus of each chain. Alpha/beta TCRs recognize antigens differently from gamma/delta TCRs.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVSIFPPSS-EQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLnswTDQDSK---DSTYSMSSTLTLTKDEYG 186
Cdd:cd05769   2 PPTVALFPPSEaEIRNKRKATLVCLATGFYPDHVSLSWKVNGKEVKDGVA---TDPQALrenTSTYSLSSRLRVSATEWF 78

                        90       100
                ....*....|....*....|....
gi 12002896 187 R-HNSYTCEATH--KTSTSPIVKS 207
Cdd:cd05769  79 NpRNTFTCIVKFygGTDTDTWTQG 102

Immunoglobulin V-set domain; This domain is found in antibodies as well as neural protein P0 and CTL4 amongst others.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896     6 QSPSSLSASLGGKVTIACKANQDINK---YIAWYQHKPGKGPRLLI-HYTSTLQPGIPS-RFSGSGSGRDYSFS--ISNL 78
Cdd:pfam07686   1 QTPREVTVALGGSVTLPCTYSSSMSEastSVYWYRQPPGKGPTFLIaYYSNGSEEGVKKgRFSGRGDPSNGDGSltIQNL 80

                          90       100
                  ....*....|....*....|....*...
gi 12002896    79 EPEDIATYYC-LQYDNLRTFGGGTKLEI 105
Cdd:pfam07686  81 TLSDSGTYTCaVIPSGEGVFGKGTRLTV 108

Immunoglobulin (Ig) variable (V) domain of T-cell receptor (TCR) beta chain; The members here are composed of the immunoglobulin (Ig) variable domain of the beta chain of alpha/beta T-cell antigen receptors (TCRs). TCRs mediate antigen recognition by T lymphocytes, and are composed of alpha and beta, or gamma and delta, polypeptide chains with variable (V) and constant (C) regions. This group includes the variable domain of the alpha chain of alpha/beta TCRs. Alpha/beta TCRs recognize antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. The variable domain of TCRs is responsible for antigen recognition, and is located at the N-terminus of the receptor. Gamma/delta TCRs recognize intact protein antigens directly without antigen processing and recognize MHC independently of the bound peptide. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896   5 TQSPSSLSASLGGKVTIACkaNQDINKY-IAWYQHKPGKGPRLLIHYTSTLQP---GIP-SRFSGSGSGRDYSF-SISNL 78
Cdd:cd05899   2 TQSPRYLIKRRGQSVTLRC--SQKSGHDnMYWYRQDPGKGLQLLFYSYGGGLNeegDLPgDRFSASRPSLTRSSlTIKSA 79

                        90       100       110
                ....*....|....*....|....*....|
gi 12002896  79 EPEDIATYYC-----LQYDNLrTFGGGTKL 103
Cdd:cd05899  80 EPEDSAVYLCasslgGGADEA-YFGPGTRL 108

Immunoglobulin (Ig) variable (V) domain of T-cell receptor (TCR) gamma chain; The members here are composed of the immunoglobulin (Ig) variable (V) domain of the gamma chain of gamma/delta T-cell receptors (TCRs). TCRs mediate antigen recognition by T lymphocytes, and are heterodimers consisting of alpha and beta chains or gamma and delta chains. Each chain contains a variable (V) and a constant (C) region. The majority of T cells contain alpha/beta TCRs, but a small subset contain gamma/delta TCRs. Alpha/beta TCRs recognize antigens as peptide fragments presented by major histocompatibility complex (MHC) molecules. Gamma/delta TCRs recognize intact protein antigens directly without antigen processing and recognize MHC independently of the bound peptide. Gamma/delta T cells can also be stimulated by non-peptide antigens such as small phosphate- or amine-containing compounds. The variable domain of gamma/delta TCRs is responsible for antigen recognition and is located at the N-terminus of the receptor. Members of this group contain the standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896   4 MTQSPSSLSASLGGKVTIACK-ANQDINK-YIAWYQHKPGKGPRLLIHYTSTLQpgiPSRFSGSGSGR----------DY 71
Cdd:cd04982   1 LEQPQLSITREESKSVTISCKvSGIDFSTtYIHWYRQKPGQALERLLYVSSTSA---VRKDSGKTKNKfearkdvgksTS 77

                        90       100       110
                ....*....|....*....|....*....|....*..
gi 12002896  72 SFSISNLEPEDIATYYCLQYDNLRT-----FGGGTKL 103
Cdd:cd04982  78 TLTITNLEKEDSATYYCAYWESGSGyyikvFGSGTKL 114

Immunoglobulin like; IG domains that cannot be classified into one of IGv1, IGc1, IGc2, IG.

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896      8 PSSLSASLGGKVTIACKANQDINKYIAWYQhkpgKGPRLLIHytstlqpgiPSRFSGSGSGRDYSFSISNLEPEDIATYY 87
Cdd:smart00410   1 PPSVTVKEGESVTLSCEASGSPPPEVTWYK----QGGKLLAE---------SGRFSVSRSGSTSTLTISNVTPEDSGTYT 67

                           90
                   ....*....|....*...
gi 12002896     88 CLQYDNLRTFGGGTKLEI 105
Cdd:smart00410  68 CAATNSSGSASSGTTLTV 85

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DR; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DR. HLA-DR is an MHC class II cell surface receptor encoded by the human leukocyte antigen complex on chromosome 6 region 6p21.31. HLA-DR is also involved in several autoimmune conditions, disease susceptibility, and disease resistance including seronegative-rheumatoid arthritis, penicillamine-induced myasthenia, schizophrenia, Goodpasture syndrome, systemic lupus erythematosus, Alzheimers, tuberculoid leprosy, and Hashimoto's thyroiditis. HLA-DR molecules are upregulated in response to signaling. HLA-DR is an alphabeta heterodimer cell surface receptor, each subunit of which contains two extracellular domains, a membrane-spanning domain, and a cytoplasmic tail. Both alpha and beta chains are anchored in the membrane. The DR beta chain is encoded by 4 loci, however no more than 3 functional loci are present in a single individual, and no more than two on a single chromosome. Sometimes an individual may only possess 2 copies of the same locus, DRB1*. The HLA-DRB1 locus is ubiquitous and encodes a very large number of functionally variable gene products (HLA-DR1 to HLA-DR17). The HLA-DRB3 locus encodes the HLA-DR52 specificity, is moderately variable and is variably associated with certain HLA-DRB1 types. The HLA-DRB4 locus encodes the HLA-DR53 specificity, has some variation, and is associated with certain HLA-DRB1 types. The HLA-DRB5 locus encodes the HLA-DR51 specificity, which is typically invariable, and is linked to the HLA-DR2 types. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 112 PTVSIFPPSSEQLTSGGAsVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYsmsSTLTLTKDEYGRHNSY 191
Cdd:cd21000   4 PKVTVYPAKTQPLQHHNL-LVCSVNGFYPGSIEVRWFRNGQEEKAGVVSTGLIQNG-DWTF---QTLVMLETVPRSGEVY 78

                        90
                ....*....|...
gi 12002896 192 TCEATHKTSTSPI 204
Cdd:cd21000  79 TCQVEHPSVTSPL 91

CH1 domain (first constant Ig domain of the heavy chain) in immunoglobulin heavy alpha, delta, epsilon, gamma, and mu chains; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the first immunoglobulin constant-1 set domain of alpha, delta, epsilon, gamma, and mu heavy chains. This domain is found on the Fab antigen-binding fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. This group belongs to the C1-set of IgSF domains, which are classical Ig-like domains resembling the antibody constant domain. C1-set domains are found almost exclusively in molecules involved in the immune system, such as in immunoglobulin light and heavy chains, in the major histocompatibility complex (MHC) class I and II complex molecules, and in various T-cell receptors.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVSIFPPSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSKDSTYSMSSTLTLTKDEYGRHNS 190
Cdd:cd04985   1 APTVFPLQSATKSQSNGPVALGCLISDYFPESITVSWQKNTNSITSGFTRTFPVVLRSGGDYSCSSQLTVPLQEWNSGEV 80

                        90
                ....*....|..
gi 12002896 191 YTCEATHKTSTS 202
Cdd:cd04985  81 YKCQVQHSASNS 92

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II beta chain. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes and they are also expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain has two globular domains (N- and C-terminal) and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 112 PTVSIFPPSSEQLTSGgASVVCFLNNFYPKDINVKWKIDGSERQNGVlNSWTDQDSKDSTYSMSSTLTLTkdeyGRH-NS 190
Cdd:cd05766   4 PSVKVSPTKTGPLEHP-NLLVCSVTGFYPAEIEVKWFRNGQEETAGV-VSTELIPNGDWTFQILVMLETT----PRRgDV 77

                        90
                ....*....|....
gi 12002896 191 YTCEATHKTSTSPI 204
Cdd:cd05766  78 YTCQVEHSSLQSPL 91

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DQ and I-A; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of human histocompatibility antigen (HLA) DQ and mouse I-A. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). I-A and I-E have the same basic features insofar as peptide loading and presentation, they differ in that each interacts with distinctly different sets of peptides, and in the incidence of deletion of their genes. A structural understanding of the similarities and differences between I-A and I-E may help with understanding their roles in peptide presentation and T cell activation. Mouse I-Ag7 has a genetic susceptibility to autoimmune diabetes due to its small, uncharged amino acid residue at position 57 of their beta chain which results in the absence of a salt bridge between beta 57 and Arg alpha 76, which is adjacent to the P9 pocket of the peptide-binding groove. Human HLA-DR, -DQ, and -DP are about 70% similar to each other. HLA-DQ (DQ) is a cell surface receptor protein found on antigen presenting cells. It is an alphabeta heterodimer of type MHC class II. The alpha and beta chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. A person often produces two alpha-chain and two beta chain variants and thus 4 isoforms of DQ. HLA-DQ is involved in the autoimmune diseases celiac disease and diabetes mellitus type. DQ is one of several antigens involved in rejection of organ transplants. DQ2 is encoded by the HLA-DQB1*02 allele group. DQ6 is encoded by the HLA-DQB1*06 allele group. DQ2 beta-chains combine with alpha-chains, encoded by genetically linked HLA-DQA1 alleles, to form the cis-haplotype isoforms. These isoforms, nicknamed DQ2.2 and DQ2.5, are also encoded by the DQA1*0201 and DQA1*0501 genes, respectively. DQ6 beta-chains combine with alpha-chains, encoded by genetically linked HLA-DQA1 alleles, to form the cis-haplotype isoforms. For DQ6, however, cis-isoform pairing only occurs with DQ1 alpha-chains. There are many haplotypes of DQ6. Susceptibility to Leptospirosis infection was found associated with undifferentiated DQ6. DQ8 is determined by the antibody recognition of beta8 and this generally detects the gene product of DQB1*0302. DQ8 is commonly linked to autoimmune disease in the human population. DQ8 is the second most predominant isoform linked to celiac disease and the DQ most linked to Type 1 diabetes. DQ8 increases the risk for rheumatoid arthritis and is linked to the primary risk locus for RA, HLA-DR4. DR4 also plays an important role in Type 1 diabetes. DQ8 is a split antigen of the DQ3 broad antigen. MHC class II molecules play a key role in the initiation of the antigen-specific immune response. They are expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice, and induced in nonprofessional APCs, such as keratinocyctes; they are expressed on the surface of activated human T cells and on T cells from other species. MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes; these peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC, and bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 110 AAPTVSIFPPSSEQLTSGGAsVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKDeygRHN 189
Cdd:cd21001   2 VEPTVTISPSRTEALNHHNL-LVCSVTDFYPGQIKVRWFRNDQEETAGVVSTPLIRNG-DWTFQILVMLEMTPQ---RGD 76

                        90
                ....*....|....*
gi 12002896 190 SYTCEATHKTSTSPI 204
Cdd:cd21001  77 VYTCHVEHPSLQSPI 91

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) I-E; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) I-E. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). I-A and I-E molecules have the same basic features insofar as peptide loading and presentation, although each interacts with distinctly different sets of peptides. They also differ in that there is a relatively high incidence of deletion of the I-E gene in both inbred strains of mice as well as wild mice and the lack of the reverse situation i.e. the deletion of I-A genes. A detailed structural understanding of the similarities and differences between I-A and the paralogous I-E could help illuminate the respective roles these molecules play in peptide presentation and T cell activation. Mouse I-Ag7 has a genetic susceptibility to autoimmune diabetes due to its small, uncharged amino acid residue at position 57 of their beta chain which results in the absence of a salt bridge between beta 57 and Arg alpha 76, which is adjacent to the P9 pocket of the peptide-binding groove. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 112 PTVSIFPPSSEQLTSGGAsVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYsmsSTLTLTKDEYGRHNSY 191
Cdd:cd20998   7 PTVTVYPTKTQPLEHHNL-LVCSVSDFYPGNIEVRWFRNGKEEKTGIVSTGLVRNG-DWTF---QTLVMLETVPQSGEVY 81

                        90
                ....*....|...
gi 12002896 192 TCEATHKTSTSPI 204
Cdd:cd20998  82 TCQVEHPSLTDPV 94

Signal-regulatory protein (SIRP) immunoglobulin-like domain 2; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig)-like domain in Signal-Regulatory Protein (SIRP), domain 2 (C1 repeat 1). The SIRPs belong to the "paired receptors" class of membrane proteins that comprise several genes coding for proteins with similar extracellular regions, but very different transmembrane/cytoplasmic regions with different (activating or inhibitory) signaling potentials. They are commonly on NK cells, but are also on many myeloid cells. Their extracellular region contains three Immunoglobulin superfamily domains, a single V-set and two C1-set IgSF domains. Their cytoplasmic tails contain either ITIMs or transmembrane regions that have positively charged residues that allow an association with adaptor proteins, such as DAP12/KARAP, containing ITAMs. There are 3 distinct SIRP members: alpha, beta, and gamma. SIRP alpha (also known as CD172a or SRC homology 2 domain-containing protein tyrosine phosphatase substrate 1/Shps-1) is a membrane receptor that interacts with a ligand CD47 expressed on many cells and gives an inhibitory signal through immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic region that interact with phosphatases SHP-1 and SHP-2. SIRP beta has a short cytoplasmic region and associates with a transmembrane adapter protein DAP12 containing immunoreceptor tyrosine-based activation motifs to give an activating signal. SIRP gamma contains a very short cytoplasmic region lacking obvious signaling motifs, but also binds CD47, but with much less affinity.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVSifPPSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSErqngVLNSWTD-QDSKDS-TYSMSST--LTLTKDEYg 186
Cdd:cd05772   4 QPLVS--GPSGRATPGQTVSFTCKSHGFSPRDITLKWFKNGNE----LSALQTTvFPEGDSvSYSVSSTvqVVLTKDDV- 76

                        90       100
                ....*....|....*....|...
gi 12002896 187 rHNSYTCEATHKTSTSPIVKSFN 209
Cdd:cd05772  77 -HSQLTCEVAHVTLQAPLRGTAN 98

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of the major histocompatibility complex (MHC) class II alpha chain. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are also expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVSIFPpsSEQLTSGGASV-VCFLNNFYPKDINVKWKIDGSERQNGVlnSWTD-QDSKDSTYSMSSTLTLTKDEygrH 188
Cdd:cd05767   2 PPEVTVFP--KSPVELGEPNTlICFVDNFFPPVINVTWLRNGQPVTDGV--SETVfLPREDHSFRKFSYLPFTPSE---G 74

                        90
                ....*....|....*...
gi 12002896 189 NSYTCEATHKTSTSPIVK 206
Cdd:cd05767  75 DIYDCRVEHWGLEEPLLK 92

Immunoglobulin (Ig)-like variable (V) domain of the Signal-Regulatory Protein (SIRP); The members here are composed of the immunoglobulin (Ig)-like domain of the Signal-Regulatory Protein (SIRP). The SIRPs belong to the "paired receptors" class of membrane proteins that comprise several genes coding for proteins with similar extracellular regions, but very different transmembrane/cytoplasmic regions with different (activating or inhibitory) signaling potentials. They are commonly on NK cells, but are also on many myeloid cells. Their extracellular region contains three immunoglobulin superfamily domains, a single V-set, and two C1-set IgSF domains. Their cytoplasmic tails that contain either ITIMs or transmembrane regions have positively charged residues that allow an association with adaptor proteins, such as DAP12/KARAP, containing ITAMs. There are 3 distinct SIRP members: alpha, beta, and gamma. SIRP alpha (also known as CD172a or SRC homology 2 domain-containing protein tyrosine phosphatase substrate 1/Shps-1) is a membrane receptor that interacts with a ligand CD47 expressed on many cells and gives an inhibitory signal through immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic region that interact with phosphatases SHP-1 and SHP-2. SIRP beta has a short cytoplasmic region and associates with a transmembrane adapter protein DAP12 containing immunoreceptor tyrosine-based activation motifs to give an activating signal. SIRP gamma contains a very short cytoplasmic region lacking obvious signaling motifs, but also binds CD47 with much less affinity. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896   3 EMTQSPSSLSASLGGKVTIACKANQDIN-KYIAWYQhkpGKGP-RLLIHytSTLQPGIPSRFSGSGSGR----DYSFSIS 76
Cdd:cd16097   1 QVIQPEKSVSVAAGESATLHCTVTSLIPvGPIQWFR---GAGPgRELIY--NQKEGHFPRVTTVSDLTKrnnmDFSIRIS 75

                        90       100       110
                ....*....|....*....|....*....|....*.
gi 12002896  77 NLEPEDIATYYCLQY------DNLRTFGGGTKLEIK 106
Cdd:cd16097  76 NITPADAGTYYCVKFrkgspdDVEFKSGAGTELSVR 111

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DP; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DP. HLA class II histocompatibility antigen, DP(W2) beta chain is a protein that in humans is encoded by the HLA-DPB1 gene. It plays a central role in the immune system by presenting peptides derived from extracellular proteins. MHC class II molecules are encoded by three different loci, HLA-DR, -DQ, and -DP, which are about 70% similar to each other. HLA-DP is an alphabeta heterodimer cell-surface receptor. Each DP subunit (alpha-subunit, beta-subunit) is composed of a alpha-helical N-terminal domain, an IgG-like beta sheet, a membrane spanning domain, and a cytoplasmic domain. The alpha-helical domain forms the sides of the peptide binding groove. The beta sheet regions form the base of the binding groove and the bulk of the molecule as well as the inter-subunit (non-covalent) binding region. Individuals carrying the MHCII allele, HLA-DP2, are at risk for chronic beryllium disease (CBD), a debilitating inflammatory lung condition caused by the reaction of CD4 T cells to inhaled beryllium. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|...
gi 12002896 132 VCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKDeygRHNSYTCEATHKTSTSPI 204
Cdd:cd21003  23 VCHVTDFYPGNIQVRWFLNGQEETAGVVSTNLIHNG-DWTFQILVMLEMTPQ---QGDVYTCQVEHPSLDSPV 91

CH1 domain (first constant Ig domain of the heavy chain) in immunoglobulin heavy epsilon and gamma chain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the first immunoglobulin constant-1 set domain of epsilon and gamma chains. It belongs to a family composed of the first immunoglobulin constant-1 set domain of alpha, delta, epsilon, gamma, and mu heavy chains. This domain is found on the Fab antigen-binding fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. This group belongs to the C1-set of IgSF domains, which are classical Ig-like domains resembling the antibody constant domain. C1-set domains are found almost exclusively in molecules involved in the immune system, such as in immunoglobulin light and heavy chains, in the major histocompatibility complex (MHC) class I and II complex molecules, and in various T-cell receptors.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 115 SIFP--PSSEQLTSGGASVVCFLNNFYPKDINVKWKidgserqNGVLNSWTD-----QDSKDsTYSMSSTLTLTKDEYGR 187
Cdd:cd21817   3 SVFPlaPCCKSTNGSSVTLGCLVTGYFPEPVTVTWN-------SGSLTSGVKtfpavLQSSG-LYTTSSQVTVPSSSWGS 74

                        90
                ....*....|....*....
gi 12002896 188 hNSYTCEATHKTSTSPIVK 206
Cdd:cd21817  75 -QTFTCNVEHKPSSTKVDK 92

CH1 domain (first constant Ig domain of the heavy chain) in immunoglobulin heavy mu chain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the first immunoglobulin constant-1 set domain of mu chains. It belongs to a family composed of the first immunoglobulin constant-1 set domain of alpha, delta, epsilon, gamma, and mu heavy chains. This domain is found on the Fab antigen-binding fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. This group belongs to the C1-set of IgSF domains, which are classical Ig-like domains resembling the antibody constant domain. C1-set domains are found almost exclusively in molecules involved in the immune system, such as in immunoglobulin light and heavy chains, in the major histocompatibility complex (MHC) class I and II complex molecules, and in various T-cell receptors.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVsiFP-PSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQdsKDSTYSMSSTLTLTKDEYGRHN 189
Cdd:cd21819   1 APTL--FPlVSCGSSTSDPVTVGCLATDFLPDSITFSWTDDNNSLTTGVKTYPSVL--TGGTYTASSQLQVPESEWKSKE 76

                ....*...
gi 12002896 190 SYTCEATH 197
Cdd:cd21819  77 NFYCKVEH 84

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain of histocompatibility antigen (HLA) I-A; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) I-A. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). I-A and I-E molecules have the same basic features insofar as peptide loading and presentation, although each interacts with distinctly different sets of peptides. They also differ in that there is a relatively high incidence of deletion of the I-E a gene in both inbred strains of mice as well as wild mice and the lack of the reverse situation i.e. the deletion of I-A genes. A detailed structural understanding of the similarities and differences between I-A and the paralogous I-E could help illuminate the respective roles these molecules play in peptide presentation and T cell activation. Mouse I-Ag7 has a genetic susceptibility to autoimmune diabetes due to its small, uncharged amino acid residue at position 57 of their beta chain which results in the absence of a salt bridge between beta 57 and Arg alpha 76, which is adjacent to the P9 pocket of the peptide-binding groove. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 111 APTVSIFPpSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSwTDQDSKDSTYSMSSTLTLTKDEygrHNS 190
Cdd:cd21006   2 APQATVFP-KSPVLLGQPNTLICFVDNIFPPVINITWLRNSKSVTDGVYET-SFLVNRDHSFHKLSYLTFIPSD---DDI 76

                        90
                ....*....|....*....
gi 12002896 191 YTCEATHKTSTSPIVKSFN 209
Cdd:cd21006  77 YDCKVEHWGLEEPVLKHWE 95

Class Ib major histocompatibility complex (MHC) immunoglobulin domain of Qa-2; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class Ib major histocompatibility complex (MHC) immunoglobulin domain of QA-2. Qa-2 is a nonclassical MHC Ib antigen, which has been implicated in both innate and adaptive immune responses, as well as embryonic development. Qa-2 has an unusual peptide binding specificity in that it requires two dominant C-terminal anchor residues and is capable of associating with a substantially more diverse array of peptide sequences than other nonclassical MHC. Class I MHC proteins bind antigenic peptide fragments and present them to CD8+ T lymphocytes. Class I molecules consist of a transmembrane alpha chain and a small chain called the beta-2-microglobulin. The alpha chain contains three extracellular domains, two of which fold together to form the peptide-binding cleft (alpha1 and alpha2), and one which has an Ig fold (alpha3). Peptide binding to class I molecules occurs in the endoplasmic reticulum (ER) and involves both chaperones and dedicated factors to assist in peptide loading. Class I MHC molecules are expressed on most nucleated cells.

                        10        20        30        40        50        60        70
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 12002896 127 GGASVVCFLNNFYPKDINVKWKIDGSERQNGvLNSWTDQDSKDSTYSMSSTLTLTkdeYGRHNSYTCEATHKTSTSPI 204
Cdd:cd21014  17 GAVTLRCWALGFYPADITLTWQLNGEELTQD-MELVETRPAGDGTFQKWASVVVP---LGKEQNYTCHVNHEGLPEPL 90

Class Ia major histocompatibility complex (MHC) immunoglobulin domain of MIC-A and MIC-B; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class Ia major histocompatibility complex (MHC) immunoglobulin domain of MIC-A and MIC-B. MIC-A and MIC-B are homologs that serve as stress-inducible antigens on epithelial and epithelially derived cells. Both serve as ligands for the widely expressed activating immunoreceptor NKG2D, a C-type lectin-like activating immunoreceptor. MIC-B is very similar in structure to MIC-A and likely interacts with NKG2D in an analogous manner. The interdomain flexibility observed in the MIC-A structures, a feature unique to MIC proteins among MHC class I proteins and homologs, is also displayed by MIC-B, with an interdomain relationship intermediate between the two examples of MIC-A structures. Mapping sequence variations onto the structures of MIC-A and MIC-B reveals patterns completely distinct from those displayed by classical MHC class I proteins, with a number of substitutions falling on positions likely to affect interactions with NKG2D, but with other positions lying distant from the NKG2D binding sites or buried within the core of the proteins. Members of the IgC family are components of immunoglobulin, T-cell receptors, CD1 cell surface glycoproteins, secretory glycoproteins A/C, and major histocompatibility complex (MHC) class I/II molecules. In immunoglobulins, each chain is composed of one variable domain (IgV) and one or more IgC domains. These names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. The IgV domain is responsible for antigen binding and the IgC domain is involved in oligomerization and molecular interactions.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 118 PP----SSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVlNSWTD-QDSKDSTYsmsSTLTLTKDEYGRHNSYT 192
Cdd:cd21017   4 PPmvnvTRSEASEGNITVTCRASGFYPWNITLSWRQDGVSLSHDT-QQWGDvLPDGNGTY---QTWVATRICQGEEQRFT 79

                        90
                ....*....|....
gi 12002896 193 CEATHKTS--TSPI 204
Cdd:cd21017  80 CYMEHSGNhsTHPV 93

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain of histocompatibility antigen (HLA) DQ and related proteins; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) DQ. MHC class II molecules are encoded by three different loci, HLA-DR, -DQ, and -DP, which are about 70% similar to each other. HLA-DQ (DQ) is a cell surface receptor protein found on antigen presenting cells. It is an alphabeta heterodimer of type MHC class II. The alpha and beta chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. A person often produces two alpha-chain and two beta chain variants and thus 4 isoforms of DQ. Two autoimmune diseases in which HLA-DQ is involved are celiac disease and diabetes mellitus type 1. DQ is one of several antigens involved in rejection of organ transplants. DQ8 is a split antigen of the DQ3 broad antigen. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 12002896 112 PTVSIFPPSSEQLTSGGaSVVCFLNNFYPKDINVKWKIDGSERQNGVlnSWTDQDSK-DSTYSMSSTLTLTKDEygrHNS 190
Cdd:cd21008   3 PEVTVFPKSPVTLGQPN-TLICLVDNIFPPVINITWLSNGHSVTEGV--SETSFLSKsDHSFLKISYLTFLPSA---DDI 76

                        90
                ....*....|....*.
gi 12002896 191 YTCEATHKTSTSPIVK 206
Cdd:cd21008  77 YDCKVEHWGLDKPLLK 92

Immunoglobulin (Ig)-like domain of platelet-derived growth factor (PDGF) receptors (R), alpha and beta; member of the I-set of IgSF domains; The members here are composed of the immunoglobulin (Ig)-like domain of platelet-derived growth factor (PDGF) receptors (R), alpha (also known as cluster of differentiation (CD) 140a), and beta (also known as CD140b). PDGF is a potent mitogen for connective tissue cells. PDGF-stimulated processes are mediated by three different PDGFs (PDGF-A,PDGF-B, and PDGF-C). PDGFRalpha binds to all three PDGFs, whereas the PDGFRbeta binds only to PDGF-B. PDGFRs alpha and beta have similar organization: an extracellular component with five Ig-like domains, a transmembrane segment, and a cytoplasmic portion having protein tyrosine kinase activity. In mice, PDGFRalpha and PDGFRbeta are essential for normal development.

                        10        20        30        40        50        60
                ....*....|....*....|....*....|....*....|....*....|....*....|....*..
gi 12002896 143 INVKWKIDGSERQNGVLNSwtdQDSKDSTYSMSSTLTLTKDEYGRHNSYTCEATHKTSTSPIVKSFN 209
Cdd:cd05861  32 VDLEWTYPGKESGRGIEPV---EEFKVPPYHLVYTLTIPSATLEDSGTYECAVTEATNDHQDEKKIN 95