Immunoglobulin (Ig) lambda light chain variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, lambda type, variable (V) domain. 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 composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: 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. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376   3 ELTQPPSVSVAPGQTARISCSGDSL--GSYFVHWYQQKPGQAPVLVIYDDSNRPSGIPERFSGSNSGNTATLTISGTQAE 80
Cdd:cd04984    1 VLTQPSSLSVSPGETVTITCTGSSGniSGNYVNWYQQKPGSAPRYLIYEDKHRPSGIPDRFSGSKSGNTASLTISGAQTE 80

                         90       100
                 ....*....|....*....|....*
gi 767259376  81 DEADYYCSAFTHNSDVFGGGTKLTV 105
Cdd:cd04984   81 DEADYYCQVWDSNSYVFGGGTKLTV 105

Immunoglobulin (Ig) lambda light chain variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, lambda type, variable (V) domain. 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 composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: 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. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376   3 ELTQPPSVSVAPGQTARISCSGDSL--GSYFVHWYQQKPGQAPVLVIYDDSNRPSGIPERFSGSNSGNTATLTISGTQAE 80
Cdd:cd04984    1 VLTQPSSLSVSPGETVTITCTGSSGniSGNYVNWYQQKPGSAPRYLIYEDKHRPSGIPDRFSGSKSGNTASLTISGAQTE 80

                         90       100
                 ....*....|....*....|....*
gi 767259376  81 DEADYYCSAFTHNSDVFGGGTKLTV 105
Cdd:cd04984   81 DEADYYCQVWDSNSYVFGGGTKLTV 105

Immunoglobulin C1-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376  114 TVFPPSSEELKeNKATLVCLISNFSPSGVTVAWKANGTPITQGVDTSNPTKEGN-KFMASSFLHLTSDQWRSHNSFTCQV 192
Cdd:pfam07654   2 YVFPPSPEELG-KPNTLTCLVTGFYPPDITVTWLKNGQEVTEGVKTTPPSPNSDwTYQLSSYLTVTPSDWESGDEYTCRV 80

                  ....
gi 767259376  193 THEG 196
Cdd:pfam07654  81 EHEG 84

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 767259376    5 TQPPSVSVAPGQTARISC---SGDSLGSYFVHWYQQKPGQAP--VLVIYDDSNRPSGIPERFS--GSNSGNTATLTISGT 77
Cdd:pfam07686   1 QTPREVTVALGGSVTLPCtysSSMSEASTSVYWYRQPPGKGPtfLIAYYSNGSEEGVKKGRFSgrGDPSNGDGSLTIQNL 80

                          90       100
                  ....*....|....*....|....*....
gi 767259376   78 QAEDEADYYCSAFTHNSDVFGGGTKLTVL 106
Cdd:pfam07686  81 TLSDSGTYTCAVIPSGEGVFGKGTRLTVL 109

Immunoglobulin (Ig) lambda light chain variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, lambda type, variable (V) domain. 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 composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: 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. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376   3 ELTQPPSVSVAPGQTARISCSGDSL--GSYFVHWYQQKPGQAPVLVIYDDSNRPSGIPERFSGSNSGNTATLTISGTQAE 80
Cdd:cd04984    1 VLTQPSSLSVSPGETVTITCTGSSGniSGNYVNWYQQKPGSAPRYLIYEDKHRPSGIPDRFSGSKSGNTASLTISGAQTE 80

                         90       100
                 ....*....|....*....|....*
gi 767259376  81 DEADYYCSAFTHNSDVFGGGTKLTV 105
Cdd:cd04984   81 DEADYYCQVWDSNSYVFGGGTKLTV 105

Immunoglobulin (Ig) light chain, kappa type, variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, kappa type, variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. 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 composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: 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 767259376   3 ELTQPP-SVSVAPGQTARISC-SGDSLGSYFVHWYQQKPGQAPVLVIYDDSNRPSGIPERFSGSNSGNTATLTISGTQAE 80
Cdd:cd04980    2 VMTQSPaSLSVSPGERVTISCkASQSISSNYLAWYQQKPGQAPKLLIYYASTLHSGVPSRFSGSGSGTDFTLTISSVEPE 81

                         90       100
                 ....*....|....*....|....*
gi 767259376  81 DEADYYCSAFTHNSDVFGGGTKLTV 105
Cdd:cd04980   82 DAAVYYCQQGYTFPYTFGGGTKLEI 106

Immunoglobulin C1-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376  114 TVFPPSSEELKeNKATLVCLISNFSPSGVTVAWKANGTPITQGVDTSNPTKEGN-KFMASSFLHLTSDQWRSHNSFTCQV 192
Cdd:pfam07654   2 YVFPPSPEELG-KPNTLTCLVTGFYPPDITVTWLKNGQEVTEGVKTTPPSPNSDwTYQLSSYLTVTPSDWESGDEYTCRV 80

                  ....
gi 767259376  193 THEG 196
Cdd:pfam07654  81 EHEG 84

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 767259376 112 TLTVFPPSSEELKENKATLVCLISNFSPSGVTVAWKANGTPITQGVDTSNPTKEGNK-FMASSFLHLTSDQWRSHNSFTC 190
Cdd:cd00098    1 TVTLLPPSPEEKGGGKVTLVCLVSGFYPKDITVTWLKNGVPLTSGVSTSSPVEPNDGtYSVTSSLTVPPSDWDEGATYTC 80

                         90
                 ....*....|....
gi 767259376 191 QVTHEGDTVEKSLS 204
Cdd:cd00098   81 VVTHESLKSPLSKT 94

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 767259376 111 PTLTVFPPSSEELKEN-KATLVCLISNFSPSGVTVAWKANGTPITQGVDT-SNPTKEGNKFMA-SSFLHLTSDQW-RSHN 186
Cdd:cd05769    3 PTVALFPPSEAEIRNKrKATLVCLATGFYPDHVSLSWKVNGKEVKDGVATdPQALRENTSTYSlSSRLRVSATEWfNPRN 82

                         90
                 ....*....|....*...
gi 767259376 187 SFTCQVTHEGDTVEKSLS 204
Cdd:cd05769   83 TFTCIVKFYGGTDTDTWT 100

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 767259376   4 LTQPP-SVSVAPGQTARISC--SGDSLGSYFVHWYQQKPGQAPVLVIYDDSN----RPSGIP-ERFSGSNSGN--TATLT 73
Cdd:cd04982    1 LEQPQlSITREESKSVTISCkvSGIDFSTTYIHWYRQKPGQALERLLYVSSTsavrKDSGKTkNKFEARKDVGksTSTLT 80

                         90       100       110
                 ....*....|....*....|....*....|....*.
gi 767259376  74 ISGTQAEDEADYYCSAFTHNSD----VFGGGTKLTV 105
Cdd:cd04982   81 ITNLEKEDSATYYCAYWESGSGyyikVFGSGTKLIV 116

Immunoglobulin (Ig) variable (V) domain of T-cell receptor (TCR) alpha chain and similar proteins; The members here are composed of the immunoglobulin (Ig) variable domain of the alpha 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 represents the variable domain of the alpha chain of TCRs and also includes the variable domain of delta chains of 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 767259376   5 TQPP-SVSVAPGQTARISCSGDSLGSYFVHWYQQKPGQAPVLVIYDDSNRPSGIPERFSgSNSGNTA---TLTISGTQAE 80
Cdd:cd04983    2 TQSPqSLSVQEGENVTLNCNYSTSTFYYLFWYRQYPGQGPQFLIYISSDSGNKKKGRFS-ATLDKSRkssSLHISAAQLS 80

                         90       100
                 ....*....|....*....|....*...
gi 767259376  81 DEADYYCSAFT-HNSD--VFGGGTKLTV 105
Cdd:cd04983   81 DSAVYFCALSEsGGTGklTFGKGTRLTV 108

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

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376     7 PPSVSVAPGQTARISCSGDSLGSYFVHWYQQKPgqapVLVIYDDsnrpsgipeRFSGSNSGNTATLTISGTQAEDEADYY 86
Cdd:smart00410   1 PPSVTVKEGESVTLSCEASGSPPPEVTWYKQGG----KLLAESG---------RFSVSRSGSTSTLTISNVTPEDSGTYT 67

                           90
                   ....*....|....*....
gi 767259376    87 CSAFTHNSDVFgGGTKLTV 105
Cdd:smart00410  68 CAATNSSGSAS-SGTTLTV 85

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 767259376   4 LTQPPSVSVAP-GQTARISCSGDSlGSYFVHWYQQKPGQAPVLVIY---DDSNRPSGIP-ERFSGS-NSGNTATLTISGT 77
Cdd:cd05899    1 VTQSPRYLIKRrGQSVTLRCSQKS-GHDNMYWYRQDPGKGLQLLFYsygGGLNEEGDLPgDRFSASrPSLTRSSLTIKSA 79

                         90       100       110
                 ....*....|....*....|....*....|..
gi 767259376  78 QAEDEADYYCSAfTHNSDV----FGGGTKLTV 105
Cdd:cd05899   80 EPEDSAVYLCAS-SLGGGAdeayFGPGTRLTV 110

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 767259376 110 TPTLTVFPPSSEEL-KENkaTLVCLISNFSPSGVTVAWKANGTPITQGV-DTSNPTKEGNKFMASSFLHLTSDqwrSHNS 187
Cdd:cd05767    2 PPEVTVFPKSPVELgEPN--TLICFVDNFFPPVINVTWLRNGQPVTDGVsETVFLPREDHSFRKFSYLPFTPS---EGDI 76

                         90
                 ....*....|
gi 767259376 188 FTCQVTHEGD 197
Cdd:cd05767   77 YDCRVEHWGL 86

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 767259376 114 TVFP-PSSEELKENKATLVCLISNFSPSGVTVAWKANGTPITQGVDTSNPTKEGNKFMASSFLHLTSDQWRSHNSFTCQV 192
Cdd:cd21819    3 TLFPlVSCGSSTSDPVTVGCLATDFLPDSITFSWTDDNNSLTTGVKTYPSVLTGGTYTASSQLQVPESEWKSKENFYCKV 82

                         90
                 ....*....|.
gi 767259376 193 THEGDTVEKSL 203
Cdd:cd21819   83 EHPGGNKEVPV 93

T cell receptor (TCR) gamma chain constant immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) constant (C) 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 antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. Gamma-delta TCRs recognize intact protein antigens; they recognize protein antigens directly and without antigen processing and 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.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376 111 PTLTVFPPSSEEL-KENKATLVCLISNFSPSGVTVAWKANGTPITQGVDTSNPTKEGNKFMASSFLHLTSDQwrSHNSFT 189
Cdd:cd07697    2 PKPTIFLPSIAETeKQKAGTYLCLLENFFPDVIKIHWREKKSDTILESQEGNTEKTKDTYMKFSWLTVPKKS--LGKEHR 79

                 ....*..
gi 767259376 190 CQVTHEG 196
Cdd:cd07697   80 CIYKHEN 86

CH3 domain (third constant Ig domain of heavy chains) in immunoglobulin heavy alpha, epsilon, and mu chains, and CH2 domain (second constant Ig domain of the gheavy chain) in immunoglobulin heavy gamma chain; member of the C1-set of Ig superfamily (IgSF) ; The members here are composed of the third immunoglobulin constant domain (IgC) of the gamma heavy chains and the second immunoglobulin constant domain (IgC) of alpha, epsilon, and mu heavy chains. This domain is found on the Fc 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.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376 111 PTLTVFPPSSEELKENK-ATLVCLISNF-SPSGVTVAW-KANGTPITqgVDTSNPTKEGNK-FMASSFLHLTSDQWRSHN 186
Cdd:cd07696    1 VSVFLIPPSPKDLFLTKsAKVTCLVVDLtSIEEVNVTWsREDGNEVL--ASTTNPEKHYNAtLSVVSTLTVCADDWDNGK 78

                 ....*...
gi 767259376 187 SFTCQVTH 194
Cdd:cd07696   79 TFKCKVTH 86

Immunoglobulin (Ig)-like variable (V) domain of Cluster of Differentiation (CD) 8 alpha chain; The members here are composed of the immunoglobulin (Ig)-like variable domain of the Cluster of Differentiation (CD) 8 alpha. The CD8 glycoprotein plays an essential role in the control of T-cell selection, maturation, and the T-cell receptor (TCR)-mediated response to peptide antigen. CD8 is comprised of alpha and beta subunits and is expressed as either an alpha/alpha or alpha/beta dimer. Both dimeric isoforms can serve as a coreceptor for T cell activation and differentiation, however they have distinct physiological roles, different cellular distributions, unique binding partners, etc. Each CD8 subunit is comprised of an extracellular domain containing a V-type Ig-like domain, a single pass transmembrane portion, and a short intracellular domain. The Ig domain of CD8 alpha binds to antibodies. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376  15 GQTARISCSGDSLGSYFVHWYQQKPGQAPV---LVIYDDSNRPSGIPE----RFSGSNSGNTATLTISGTQAEDEADYYC 87
Cdd:cd05720   13 GQKVELVCEVLNSVPQGCSWLFQPRGSAPQptfLLYLSSSNKTKWAEGldskRFSGSRSGSSYVLTLKDFRKEDEGYYFC 92

                 ..
gi 767259376  88 SA 89
Cdd:cd05720   93 SV 94

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 767259376 108 KSTPTLTVFPPSSEELkENKATLVCLISNFSPSGVTVAWKANGTPITQGVDTSNPTKEGN-KFMASSFLHLTSdqwRSHN 186
Cdd:cd05766    1 RVQPSVKVSPTKTGPL-EHPNLLVCSVTGFYPAEIEVKWFRNGQEETAGVVSTELIPNGDwTFQILVMLETTP---RRGD 76

                 ....*...
gi 767259376 187 SFTCQVTH 194
Cdd:cd05766   77 VYTCQVEH 84

Class II major histocompatibility complex (MHC) alpha 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 immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) DR. MHC class II molecules are encoded by three different loci, HLA-DR, -DQ, and -DP, which are about 70% similar to each other. HLA-DR 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-DRA1 and HLA-DRB1, that are adjacent to each other on chromosome band 6p21.31. Susceptibility to multiple sclerosis and rheumatoid arthritis are associated with the human histocompatibility leukocyte antigen HLA-DR2 and HLA-DR4, respectively. 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 767259376 111 PTLTVFPPSSEELKENKaTLVCLISNFSPSGVTVAWKANGTPITQGV-DTSNPTKEGN---KFMASSFLHLTSDqwrshn 186
Cdd:cd21007    3 PEVTVLTNSPVELREPN-VLICFIDKFTPPVVNVTWLRNGKPVTTGVsETVFLPREDHlfrKFHYLPFLPSTED------ 75

                         90
                 ....*....|
gi 767259376 187 SFTCQVTHEG 196
Cdd:cd21007   76 VYDCRVEHWG 85

HLA class II histocompatibility antigen DO alpha; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the nonclassical MHC class II (MHCII) protein, HLA-DO, which binds HLA-DM and influences the repertoire of peptides presented by MHCII proteins. In complex with HLA-DM, HLA-DO adopts a classical MHCII structure, with alterations near the a subunit's 310 helix. HLA-DO binds to HLA-DM at the same sites implicated in MHCII interaction, and kinetic analysis showed that HLA-DO acts as a competitive inhibitor by acting as a substrate mimic. Though more remains to be elucidated about the function of HLA-DO, its unique distribution in the mammalian body namely, the exclusive expression of HLA-DO in B cells, thymic medullary epithelial cells, and dendritic cells indicate that it may be of physiological importance and has inspired further research. 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        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376 111 PTLTVFPPSSEELKENKaTLVCLISNFSPSGVTVAWKANGTPITQGVDT----SNPTKEGNKFMASSFLHLTSDqwrshn 186
Cdd:cd21004    3 PRVTVLPKSRVELGQPN-ILICIVDNIFPPVINITWLRNGQTVTEGVAQtsfySQPDHLFRKFHYLPFVPSAED------ 75

                         90
                 ....*....|
gi 767259376 187 SFTCQVTHEG 196
Cdd:cd21004   76 VYDCKVEHWG 85

Class II major histocompatibility complex (MHC) alpha 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 immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) I-E. 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 767259376 111 PTLTVFPPSSEELKENKaTLVCLISNFSPSGVTVAWKANGTPITQGVDTSN--PTKEG--NKFMASSFLHLTSDqwrshn 186
Cdd:cd21005    3 PEVTVLSRSPVNLGEPN-ILICFIDKFSPPVVNVTWLRNGRPVTEGVSETVflPRDDHlfRKFHYLTFLPSTDD------ 75

                         90
                 ....*....|
gi 767259376 187 SFTCQVTHEG 196
Cdd:cd21005   76 FYDCEVDHWG 85

Signal-regulatory protein (SIRP) immunoglobulin-like domain 3; 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 3 (C1 repeat 2). 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 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 767259376 111 PTLTVFPPSSEelKENKATLVCLISNFSPSGVTVAWKANGTpITQGVDTSNPT--KEGNKFMASSFLHLTSDQwRSHNSF 188
Cdd:cd16085    3 PTLEVTQQPTM--VWNQVNVTCQVEKFYPQRLQLTWLENGN-VSRTETPSTLTvnKDGTYNWTSWLLVNVSAH-REDVVL 78

                 ....*...
gi 767259376 189 TCQVTHEG 196
Cdd:cd16085   79 TCQVEHDG 86

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 767259376 111 PTLTVFPPSSEELKENKaTLVCLISNFSPSGVTVAWKANGTPITQGVDTSNPTKEGN-KFMASSFLHLTSDQwrsHNSFT 189
Cdd:cd21001    4 PTVTISPSRTEALNHHN-LLVCSVTDFYPGQIKVRWFRNDQEETAGVVSTPLIRNGDwTFQILVMLEMTPQR---GDVYT 79

                 ....*
gi 767259376 190 CQVTH 194
Cdd:cd21001   80 CHVEH 84

Class Ib major histocompatibility complex (MHC) immunoglobulin domain of Qa-1b; member of the C1-set of Ig superfamily (IgSF) domains; The non-classical mouse MHC class I (MHC-I) molecule Qa-1b is a non-polymorphic MHC molecule with an important function in innate immunity. It binds and presents signal peptides of classical MHC-I molecules at the cell surface and, as such, act as an indirect sensor for the normal expression of MHC-I molecules. This signal peptide dominantly accommodated in the groove of Qa-1b is called Qdm, for Qa-1 determinant modifier, and its amino acid sequence AMAPRTLLL is highly conserved among mammalian species. The Qdm/Qa-1b complex serves as a ligand for the germ-line encoded heterodimeric CD94/NKG2A receptors expressed on natural killer (NK) cells and activated CD8+ T cells and transduces inhibitory signals to these lymphocytes. Thus, upon binding, Qa-1b signals NK cells not to engage in cell lysis. The molecular basis of Qa-1b function is unclear.

                         10        20        30        40        50        60        70
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 767259376 125 ENKATLVCLISNFSPSGVTVAWKANGTPITQGVD--TSNPTKEGN-KFMASSFLHLTSDQWrshnsFTCQVTHEG 196
Cdd:cd21820   17 EDEVTLRCWALGFYPADITLTWQLNGEELTQDMElvETRPAGDGTfQKWAAVVVPLGKEQY-----YTCHVYHEG 86

Class Ia major histocompatibility complex (MHC) immunoglobulin domain of RT1-Aa; 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 RT1-Aa. While most mammalian species transport these peptides into the ER via a single allele of TAP, rats have evolved different TAPs, TAP-A and TAP-B, RT1-Aa and RT1-A1c, which are associated with TAP-A and TAP-B. The rat MHC class Ia molecule RT1-Aa has the unusual capacity to bind long peptides ending in arginine, such as MTF-E, a thirteen-residue, maternally transmitted minor histocompatibility antigen. 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        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376 107 PKSTPTLTVFPpsseelkENKATLVCLISNFSPSGVTVAWKANGTPITQGVD--TSNPTKEGN-KFMASSFLHLTSDQwr 183
Cdd:cd21015    5 PEAHVTLHPRP-------EGDVTLRCWALGFYPADITLTWQLNGEDLTQDMElvETRPAGDGTfQKWASVVVPLGKEQ-- 75

                         90
                 ....*....|...
gi 767259376 184 shnSFTCQVTHEG 196
Cdd:cd21015   76 ---NYTCRVEHEG 85

Immunoglobulin variable domain (IgV) Cluster of Differentiation (CD) 79B; The members here are composed of the immunoglobulin variable domain (IgV) of the Cluster of Differentiation (CD) 79B (also known as CD79b molecule, immunoglobulin-associated beta (Ig-beta), and B29). The B lymphocyte antigen receptor is a multimeric complex that includes the antigen-specific component, surface immunoglobulin (Ig). Surface Ig non-covalently associates with two other proteins, Ig-alpha and Ig-beta, which are necessary for expression and function of the B-cell antigen receptor. This gene encodes the Ig-beta protein of the B-cell antigen component. Alternatively spliced transcript variants encoding different isoforms have been described. Members of the IgV family are components of immunoglobulin (Ig) and T cell receptors. 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. Within the variable domain, there are regions of even more variability called the hypervariable or complementarity-determining regions (CDRs) which are responsible for antigen binding. A predominant feature of most Ig domains is the disulfide bridge connecting 2 beta-sheets with a tryptophan residue packed against the disulfide bond. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 767259376  15 GQTARISCSGDSLGSyfVHWYQQKPGQAPVLVIYDDSnrpsgipeRFSGSNSGNTATLTISGTQAEDEADYYCSAFTHNS 94
Cdd:cd16096   13 SSMVKFHCYTNYSGV--MTWFRKKGNQRPQELFPEDG--------RISQTQNGSVYTLTIQNIQYEDNGIYFCQQKCNST 82

                         90
                 ....*....|....
gi 767259376  95 --DVFGG-GTKLTV 105
Cdd:cd16096   83 epDVTDGcGTELLV 96

Class Ia major histocompatibility complex (MHC) immunoglobulin domain of human leukocyte antigen (HLA) H2Db and H2Ld; 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 human leukocyte antigen (HLA) H2Db and H2Ld. H-2Ld complexed with peptide QL9 (or p2Ca) and complexed with influenza virus peptide NP366-374 (ASNEN-METM), respectively are high-affinity alloantigens for the 2C T cell receptor (TCR). The a1-a2 super domains of H-2Ld, H-2Db, and H-2Kb closely superimpose. 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 767259376 125 ENKATLVCLISNFSPSGVTVAWKANGTPITQGVD--TSNPTKEGN-KFMASSFLHLTSDQwrshnSFTCQVTHEG 196
Cdd:cd21018   17 KGEVTLRCWALGFYPADITLTWQLNGEELTQDMElvETRPAGDGTfQKWASVVVPLGKEQ-----NYTCRVYHEG 86

Class Ib major histocompatibility complex (MHC) immunoglobulin domain of T10, T22, and similar proteins; 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 the murine H-2T-encoded T10, T22, and similar proteins. T10 and T22 are highly related nonclassical major histocompatibility complex (MHC) class Ib proteins that bind to certain gammadelta T cell receptors (TCRs) in the absence of other components. Classical MHC class I (class Ia) molecules participate in immune responses by presenting peptide antigens to cytolytic alpha beta T cells. Many nonclassical MHC class I (class Ib) molecules have distinct antigen-binding capabilities, suggesting that they have evolved for specific tasks that are distinct from those of MHC class Ia. 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
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 767259376 125 ENKATLVCLISNFSPSGVTVAWKANGTPITQGVD--TSNPTKEGN-KFMASSFLHLTSDQwrshnSFTCQVTHEG 196
Cdd:cd21016   17 EGDVTLRCWALGFYPADITLTWQKDGEELTQDMEfvETRPAGDGTfQKWAAVVVPLGKEQ-----SYTCHVYHEG 86

Class Ia major histocompatibility complex (MHC) immunoglobulin domain of H2-Dd; 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 H2-Dd. Mouse MHC is composed of 11 subclasses. It includes the classical MHC class I (MHC-Ia) that comprises H-2D, H-2K and H-2L subclasses, the non-classical MHC class I (MHCIb) that comprises H-2Q, H-2M and H-2T subclasses, the classical MHC class II (MHC-IIa) that includes H-2A(I-A) and H-2E(I-E) subclasses, and the non-classical MHC class II (MHC-IIb) comprises H-2M and H-2O. H-2K, H-2D, and H-2L are 80 to 90% homologous at the amino acid level yet appear to be involved in different recognition reactions and are differentially expressed on lymphoid cells. 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 767259376 125 ENKATLVCLISNFSPSGVTVAWKANGTPITQGVD--TSNPTKEGN-KFMASSFLHLTSDQwrshnSFTCQVTHEG 196
Cdd:cd21020   17 EGDVTLRCWALGFYPADITLTWQLNGEELTQEMElvETRPAGDGTfQKWASVVVPLGKEQ-----KYTCHVEHEG 86