Lymphoid System

Cells of the lymphoid system provide highly specific protection against foreign agents and also orchestrate the functions of other parts of the immune system by producing immunoregulatory cytokines. The lymphoid system is divided into 1) central lymphoid organs, the thymus and bone marrow, and 2) peripheral lymphoid organs, lymph nodes, the spleen, and mucosal and submucosal tissues of the alimentary and respiratory tracts. The thymus instructs certain lymphocytes to differentiate into thymus-dependent (T) lymphocytes and selects most of them to die in the thymus (negative selection) and others to exit into the circulation (positive selection). T lymphocytes circulate through the blood, regulate antibody and cellular immunity and help defend against many types of infections. The other classes of lymphocytes, B cells (antibody-forming cells) and natural killer (NK) cells, are thymic-independent and remain principally in peripheral lymphoid organs.

Reticuloendothelial System

Cells of the RES provide natural immunity against microorganisms by 1) a coupled process of phagocytosis and intracellular killing, 2) recruiting other inflammatory cells through the production of cytokines, and 3) presenting peptide antigens to lymphocytes for the production of antigen-specific immunity. The RES consists of 1) circulating monocytes; 2) resident macrophages in the liver, spleen, lymph nodes, thymus, submucosal tissues of the respiratory and alimentary tracts, bone marrow, and connective tissues; and 3) macrophage-like cells including dendritic cells in lymph nodes, Langerhans cells in skin, and glial cells in the central nervous system.

Myeloid Cells

Neutrophils: These are one of the major types of cells that are recruited to ingest, kill, and digest pathogens. Neutrophils are the most highly adherent, motile, phagocytic leukocytes and are the first cells recruited to acute inflammatory sites. Each of their functions is dependent upon special proteins, such as the adherence molecule CD11b/CD18, or biochemical pathways, such as the respiratory burst associated with cytochrome b₅₅₈.

Eosinophils: Eosinophils defend against many types of parasites and participate in common hypersensitivity reactions via cytotoxicity. That cytotoxicity is mediated by large cytoplasmic granules, which contain the eosinophilic basic and cationic proteins.

Basophils: These cells and their tissue counterparts, mast cells, produce cytokines that help defend against parasites and engender allergic inflammation. These cells display high affinity surface membrane receptors for IgE antibodies and have many large cytoplasmic granules, which contain heparin and histamine. When cell-bound IgE antibodies are cross-linked by antigens, the cells degranulate and produce low-molecular weight vasoactive mediators (e.g. histamine) through which they exert their biological effects.

Monocytes/Macrophages: Monocytes and macrophages are involved in phagocytosis and intracellular killing of microorganisms. Macrophages process protein antigens and present peptides to T cells. These monocytes/macrophages are highly adherent, motile and phagocytic; they marshal and regulate other cells of the immune system, such as T lymphocytes; serve as antigen processing-presenting cells; and act as cytotoxic cells when armed with specific IgG antibodies.

Macrophages are differentiated monocytes, which are one of the principal cells found to reside for long periods in the RES. Macrophages may also be recruited to inflammatory sites, and be further activated by exposure to certain cytokines to become more effective in their biologic functions.

Lymphoid Cells

These cells provide efficient, specific and long-lasting immunity against microbes and are responsible for acquired immunity. Lymphocytes differentiate into three separate lines: thymic-dependent cells or T lymphocytes that operate in cellular and humoral immunity, B lymphocytes that differentiate into plasma cells to secrete antibodies, and natural killer (NK) cells. T and B lymphocytes are the only lymphoid cells that produce and express specific receptors for antigens.

T Lymphocytes: These cells are involved in the regulation of the immune response and in cell mediated immunity and help B cells to produce antibody (humoral immunity). Mature T cells express antigen-specific T cell receptors (TcR) that are clonally segregated (i.e., one cell lineage-one receptor specificity). Every mature T cell also expresses the CD3 molecule, which is associated with the TcR. In addition mature T cells display one of two accessory molecules, CD4 or CD8. The TcR/CD3 complex recognizes antigens associated with the major histocompatibility complex (MHC) molecules on target cells (e.g. virus-infected cell). The TcR is a transmembrane heterodimer composed of two polypeptide chains (usually, α and β chains). Each chain consists of a constant (C) and a variable (V) region, and are formed by a gene-sorting mechanism similar to that found in antibody formation. The repertoire is generated by combinatorial joining of variable (V), joining (J), and diversity (D) genes, and by N region (nucleotides inserted by the enzyme deoxynucleotidyl-transferase) diversification. Unlike immunoglobulin genes, genes encoding TcR do not undergo somatic mutation. Thus there is no change in the affinity of the TcR during activation, differentiation, and expansion.

T Helper Cells: These cells are the primary regulators of T cell- and B cell-mediated responses. They 1) aid antigen-stimulated subsets of B lymphocytes to proliferate and differentiate toward antibody-producing cells; 2) express the CD4 molecule; 3) recognize foreign antigen complexed with MHC class II molecules on B cells, macrophages or other antigen-presenting cells; and 4) aid effector T lymphocytes in cell-mediated immunity. Currently, it is believed that there are two functional subsets of T helper (Th) cells. Th1 cells aid in the regulation of cellular immunity, and Th2 cells aid B cells to produce certain classes of antibodies (e.g., IgA and IgE). The functions of these subsets of Th cells depend upon the specific types of cytokines that are generated, for example interleukin-2 (IL-2) and interferon-γ (IFN-γ) by Th1 cells and IL-4 and IL-10 by Th2 cells.

Cell-mediated immunity (delayed hypersensitivity) plays an important role in defense against many intracellular infections such as Mycobacterium tuberculosis. This inflammatory reaction is initiated by the recognition of specific antigens by Th1 cells. Consequently, lymphokines are generated which recruit activated macrophages to eliminate foreign antigens or altered host cells.

T Cytotoxic Cells: These cells are cytotoxic against tumor cells and host cells infected with intracellular pathogens. These cells 1) usually express CD8, 2) destroy infected cells in an antigen-specific manner that is dependent upon the expression of MHC class I molecules.

T Suppressor Cells: These cells suppress the T and B cell responses and express CD8 molecules.

Natural Killer Cells: NK cells are large granular lymphocytes that nonspecifically kill certain types of tumor cells and virus-infected cells. Killing by NK cells is enhanced by cytokines such as IL-2 and IFN-γ. NK cells are also activated by microorganisms to produce a number of cytokines [(IL-2, IFN-γ, IFN-α, and tumor necrosis factor-α (TNF-α)]. These circulating large granular lymphocytes do not express CD3, TcR or immunoglobulin, but display surface receptors (CD16) for the Fc fragment of IgG antibodies.

B Lymphocytes: These cells differentiate into plasma cells to secrete antibodies and are involved in processing proteins and presenting the resultant peptide antigen fragments in the context of MHC molecule to T cells. The genesis of μ and δ chain-positive, mature B cells from pre-B cells is antigen-independent. Pre-B cells in the bone marrow undergo gene rearrangement for IgM heavy (H) chains and consequently express those proteins in the cytoplasm (the μ chain), but no immunoglobulin light (L) chains. B cell development is characterized by recombinations of immunoglobulin H and L chain genes and expression of specific surface monomeric IgM molecules. At this stage of development, B cells are highly susceptible to the induction of tolerance. Once these cells acquire IgD molecules on their surface, they become mature B cells that are able to differentiate after exposure to antigen into antibody-producing plasma cells.

The activation of B cells into antibody producing/secreting cells (plasma cells) is antigen-dependent. Once specific antigen binds to surface Ig molecule, the B cells differentiate into plasma cells that produce and secrete antibodies of the same antigen-binding specificity. If B cells also interact with Th cells, they proliferate and switch the isotype (class) of immunoglobulin that is produced, while retaining the same antigen-binding specificity. This occurs as a result of recombination of the same Ig VDJ genes (the variable region of the Ig) with a different constant (C) region gene such as IgG. In the case of protein antigens, Th2 cells are thought to be required for switching from IgM to IgG, IgA, or IgE isotypes.

IgM is therefore the principal antibody produced during a primary immunization. This primary antibody response is manifested by serum IgM antibodies as early as 3–5 days after the first exposure to an immunogen (immunizing antigen), peaks in 10 days, and persists for some weeks. Secondary or anamnestic antibody responses following repeated exposures to the same antigen appear more rapidly, are of longer duration, have higher affinity, and principally are IgG molecules.

When antibodies bind to antigens, they may 1) neutralize pathogenic features of antigens such as their toxins, 2) facilitate their ingestion by phagocytic cells (opsonization), 3) fix to and activate complement molecules to produce opsonins and chemoattractants (vide infra), or 4) participate in antibody-dependent cellular cytotoxicity (ADCC).

In addition to antibody formation, B cells also process and present protein antigens. After the antigen is internalized it is digested into fragments, some of which are complexed with MHC class II molecules and then presented on the cell surface to CD4⁺ T cells.

Immunoglobulins (Ig)/Antibodies

Immunoglobulins (antibodies) are globular glycoproteins found in body fluids or on B cells where they act as antigen receptors. These molecules are either expressed on the surface of B cells or are secreted by terminally differentiated cells from this lineage (plasma cells) into the circulation or external secretions. An immunoglobulin molecule is a symmetrical multi-chain peptide consisting of two identical H chains and two identical L chains. Each chain is divided into a V region that is responsible for specific antigen binding and a C region that carries out other functions such as the binding of IgG to complement or leukocytes. These antibody molecules are formed as a result of the assembly of separate germ-line genes for the V, J, and C regions of the H and L chains of the final immunoglobulin molecule. This combinatorial mechanism is responsible for the great diversity of antibody molecules.

There are five major isotypes (classes) of immunoglobulins (IgG, IgA, IgM, IgD, and IgE). These isotypes are distinguished by differences in the C regions of H chains of each immunoglobulin isotype (γ, α, μ, δ, and ε, respectively). These differences are responsible for the particular functions of immunoglobulin classes.

T Cell Receptor

The specific receptor for antigen on T lymphocytes, the TcR, is a heterodimeric protein with motifs that are similar to immunoglobulin molecules, but whose structure is encoded by a different set of V, J, D, and C genes. Moreover, T cells consist of two subsets carrying different receptors, that have been designated α/β and γ/δ. The T cell receptors act as specific antigen recognition molecules. Unlike antibody molecules, the TcR molecules cannot recognize soluble antigens. In contrast, they recognize protein antigens that have been processed and presented as peptides on the surface of antigen-presenting cells in the context of MHC class I or MHC class II molecules (vide infra).

Major Histocompatibility Complex (MHC)

These genes encode for cell surface molecules that are involved in the genesis and regulation of specific immune responses to T-cell dependent antigens and in tissue transplantation. They principally encode cell surface protein molecules that bind antigenic peptides, which are recognized by T cells.

The MHC is a cluster of ~ 40–50 genes located on chromosome 6. These genes belong to the super-immunoglobulin gene family. There are three classes of these molecules. MHC class I molecules are found on all nucleated somatic cells and aid in presenting endogenously synthesized antigens, whereas MHC class II molecules are found principally on antigen processing/presenting cells (i.e., macrophages, B cells) and are involved in presenting processed exogenous protein antigens. The MHC class III region contains a heterogeneous group of genes that encode for some components of the complement system, heat shock proteins, tumor necrosis factor-α, and tumor necrosis factor-β.

Neutrophils

Neutrophils are the first circulating phagocytic cells recruited to the site of infection and inflammation to ingest, kill, and digest pathogens. These cells are produced from myeloid stem cells in the bone marrow (Fig. 1-2). Neutrophils constitute the large number of leukocytes in the blood. After stimulation, mature neutrophils display more motility, adherence, phagocytic activity, and intracellular killing than any other type of cell (Fig. 1-5). Neutrophils persist in the circulation for only several hours. Then, they are either removed by the RES or migrate into inflammatory sites.

Figure 1-5

Development and function of neutrophils.

Eosinophils

Eosinophils play a major role in the killing of parasites, particularly hemoflagellates, echinococcus, and enteric nematodes. This killing is due to a basic protein and a cationic protein contained in large cytoplasmic granules that are unique to eosinophils. These cells also play a prominent role in the pathogenesis of the allergic inflammation. These cells are induced to grow and differentiate by interleukin-5 and are recruited to inflammatory sites by agents such as platelet-activating factor from the lipoxygenase segment of the arachidonic acid pathway.

Basophils

Basophils and their tissue counterparts, the mast cells, play a major role in defense against parasites and in allergic inflammation. These cells are distinguished by many large cytoplasmic granules that contain heparin and histamine and by high affinity receptors for IgE antibodies. If these cell bound IgE antibodies are cross-linked by antigens, the cells degranulate and are activated to produce and secrete a group of low-molecular weight vasoactive mediators and certain proinflammatory cytokines, e.g. tumor necrosis factor α (TNF-α) and interleukin-5 (IL-5).

Monocytes and Macrophages

Some functions of macrophages such as phagocytosis and intracellular killing are similar to those of neutrophils, whereas others are distinct. The distinctive features are as follows: a) They are able to reside in the RES for long periods. b) They process protein antigens and present the resultant peptide fragments to T cells in the context of MHC class II molecules. They produce cytokines (Table 1-2). These cells are also highly adherent, motile and phagocytic. These properties are greatest in activated macrophages, somewhat less in unstimulated macrophages, and least in monocytes. The role of these cells in processing and presenting antigens is dealt with in the next section.

Monocytes and macrophages are activated by bacterial products such as endotoxin (lipopolysaccharides); autocrine agents, such as TNF-α, IL-1, and IL-8; cytokines such as interferon-γ (IFN-γ) and a special group of mediators called chemokines. Activated macrophages play a prominent effector role in cellular immunity by 1) ingesting and killing pathogens, 2) clearing immune complexes, and 3) aiding in the genesis of specific immune responses by antigen presentation.

T Lymphocytes

T lymphocytes are thymus-derived lymphocytes and play a central role in the generation and regulation of the immune response to protein antigens. T cells originate from bone marrow stem cells (Fig. 1-2) that develop into T precursor cells that migrate to the thymus where they multiply and differentiate (Fig. 1-6). The rate at which the thymus produces T cells is very high in childhood and declines thereafter. Because mature T cells are long lived and recirculate (Fig. 1-7), they comprise about 70–80% of lymphocytes in blood and lymph, and they are responsible for much of the immunologic memory.

Figure 1-6

Ontogeny of B and T lymphocytes.

Figure 1-7

Lymphocyte circulation pathways. T cells are principally recirculating; B cells are principally sequestered in peripheral lymphoid organs.

Maturation

The maturation of T cells takes place in the thymus and is characterized by a sequential appearance of certain cell surface molecules. Among the first surface molecules to appear are CD3, T cell receptors (TcR), which are α/β positive (Fig. 1-8) ; CD4; and CD8. Thus immature thymocytes are CD3⁺CD4⁺CD8⁺. Cortical T cells lose either CD4 or CD8 molecules to become CD3⁺TcR⁺CD4⁺ or CD3⁺TcR⁺CD8⁺. Mature T cells migrate to the medulla of the thymus from where they exit into the systemic circulation.

Figure 1-8

The TcR-CD3 complex on helper (CD4⁺) or cytotoxic/suppressor (CD8⁺) T cells. The TcR receives peptide fragments from antigen presenting cells. CD3 is a signaling molecule.

The TcR recognizes protein antigen determinants that are presented by MHC molecules (see below). In addition, TcR are physically associated with CD3. This association with CD3 is required for transmembrane signaling that culminates in T cell activation.

Role of MHC in T Cell Development

One major function of MHC molecules is to present antigens to T cells. Lymphocytes in the thymus are exposed to various endogenous (self) proteins, particularly the products of MHC (see below). Some nascent T cells that have specificity towards self MHC molecules are eliminated (negative selection), while remaining T cells become “educated” to recognize foreign antigenic peptides that are associated with self MHC (positive selection). Thus, antigen recognition by T cells becomes “MHC restricted,” that is, the mature T cell recognizes its specific antigen only if that antigen is presented by the correct MHC molecule.

Two kinds of MHC genes, class I and class II (see Fig. 1-9 for their protein products) (see section on MHC), are involved in the development of T cells. In the course of selective adaptation, T cells learn to recognize foreign antigens in association with protein products of either MHC class I or II genes. MHC class I-restricted T cells express CD8 molecules that bind to the invariant portion of MHC class I, whereas MHC class II-restricted T cells express the CD4 molecule that binds to MHC class II molecule. Thus, mature T lymphocytes leaving the thymus are either CD4⁺ or CD8⁺ (single positive) and express CD3 and TcR molecules.

Figure 1-9

Structures of MHC class I and II molecules. Binding sites in the molecules are shown for processed protein antigens for presentation to T cells. Leters N and C represent N and C termini of the polypeptide, respectively.

T-Cell Subpopulations

Both CD4 and CD8 molecules participate in T cell activation. CD4⁺ T cells are principally regulatory cells, which control the functions of other lymphocytes. Based on the lymphokines they produce, CD4⁺ Th cells are divided into two subsets, namely Th1 cells that promote cellular immunity (Fig. 1-10), and Th2 cells that help antibody production (Fig. 1-11). CD8⁺ T cells are cytotoxic/suppressor cells which participate in cell-mediated immunity against viruses, fungi, bacteria, and against certain tumors and play a role in immune regulation.

Figure 1-10

Genesis of cellular immunity and T-cytotoxic cells by activation of Th1 cells.

Figure 1-11

Immunoglobulin isotype switching. Reconfiguration of genes for IgM to IgA while retaining the same antigen binding specificity. According to which switch sites combine, the intervening DNA is looped out and eventually deleted. In this illustration, an (more...)

T Helper (Th) Cells

These cells are involved in the regulation of both T cell and B cell- mediated immune responses. IgG, IgA, and IgE antibody responses against T-dependent antigens require Th2 cells. Th2 cells aid antigen-activated B cells to proliferate and differentiate into antibody-producing plasma cells and to undergo class switching (Fig. 1-11). They recognize foreign antigens complexed with MHC class II molecules on antigen-presenting cells (B cells, macrophages, dendritic cells and Langerhans cells).

Antigens are presented to Th2 cells in two ways. In the first (Fig. 1-12), the antigen is taken up and processed by accessory cells, such as macrophages or B cells, that present the Ag/MHC complex to Th2 cells. Activated T cells then produce lymphokines that recruit and activate B cells to produce antibodies. Unlike phagocytic cells, B cells bind the antigen by specific antibodies, then they internalize and process the antigen (Ag), and express a fragment of it bound to MHC class II molecules on the cell surface in the context of MHC class II molecules. Antigen-specific Th2 cells that bind the Ag/MHC complex on the antigen-presenting cells become activated and produce helper factors for adjacent B cells. Furthermore, macrophages may process and present antigens without MHC products to B cells or, in the case of complex polysaccharides, the antigen may be presented directly to B cells (Fig. 1-12) without the aid of other cells. Which pathway is used depends on the nature of the antigen.

Figure 1-12

Antigen presentation mechanisms.

Helper T cells (Th1) also aid effector T lymphocytes (vide infra) in cell-mediated immunity. This process occurs according to the pathway depicted in Figure 1-12, except that the recipients of the helper factor are effector T cells.

B and T cells require different cytokines for growth and differentiation. The pattern of the production of those particular factors define whether the cells are Th1 or Th2. For example, Th1 cells produce IFN-γ, a cytokine that activates macrophages. Those activated macrophages in turn participate in delayed hypersensitivity, a major aspect of cell-mediated immunity (Fig. 1-10). In contrast, Th2 cells produce cytokines such as IL-4 and IL-10, which activate certain phases of antibody production and inhibit the genesis of delayed hypersensitivity.

Natural Killer Lymphocytes

These cells provide innate protection by killing tumor cells and cells infected with intracellular pathogens. Natural killer cells are large granular lymphocytes that do not express CD3, TcR, or immunoglobulins (Table 1-1), but display a low-affinity surface receptor for the Fc fragment of IgG (CD16; e.g. CR3) and CD56 (Fig. 1-3). Natural killer cells account for 10–15% of blood lymphocytes and are found in low numbers in the peripheral lymphoid system.

Natural killer cells regulate certain aspects of T and B cell activation and hematopoiesis, and they defend against certain tumors and intracellular infections by killing the involved cells. In contrast to cytotoxic T cells, the NK cell-mediated cytotoxicity neither requires previous sensitization nor is MHC-restricted. The cytotoxicity of NK cells is increased after exposure to cytokines such as IL-2 or IFN-γ. NK cells also mediate antibody-dependent, cell-mediated cytotoxicity via the CD16 Fc receptor. NK cells can be activated to produce cytokines (IL-2, IFN-γ, IFN-α, TNF-α) that aid in immunomodulation.

B Lymphocytes

These cells are primarily involved in antibody production and antigen presentation to T cells. B cells originate from lymphoid stem cells in the fetal liver and the bone marrow (Fig. 1-2). B lymphocytes are thymus-independent cells that express intrinsically produced immunoglobulins (vide infra) on their external membranes and upon stimulation by antigen differentiate into plasma cells that produce and secrete large numbers of antibody molecules (Fig. 1-6). Pre-B cells, the immediate precursors of B cells, are restricted to the bone marrow and are characterized by the presence of cytoplasmic μ chains (H chains for IgM) but no L chains. Mature but unstimulated B cells express monomeric IgM antibodies, MHC class II molecules, CD19, CD20, the Epstein-Barr virus/C3d (CR2) (CD21) receptor, and T cell interaction molecules, B7-1, B7-2, and the CD40 ligand, CD39 (Fig. 1-13). B cells account for 10–15% of blood lymphocytes. They, and their progeny, antibody-producing cells, primarily reside in peripheral lymphoid organs.

Figure 1-13

Surface markers on B cells.

Each B cell expresses and produces immunoglobulin molecules of one antigen-binding specificity. Clones expressing different specificities are involved in the production of antibodies to a complex immunogen because of the multiplicity of antigenic determinants (epitopes) on the molecules. Hence, many separate clones of B cells are required to produce the overall antibody response (a polyclonal response). If the immunogen has a very limited set of epitopes, the antibody response will be oligoclonal or monoclonal.

Isotype Switching

B lymphocytes switch their immunoglobulin production from IgM to IgG, IgA, or IgE, during the course of immune response against T cell-dependent antigens. Lymphokines from T helper cells are necessary for the class (isotype) switch that occurs in antigen-stimulated B cells. These events in B-cell differentiation are accompanied by immunoglobulin gene rearrangements, which will be described later in this chapter (Fig. 1-14). As a result of the recombination of the same VDJ genes with a different C region gene, a different isotype of immunoglobulin with the same antibody specificity is produced (Fig. 1-11). Once the mature B cell encounters the appropriate antigen, the cell differentiates to form a plasma cell. Plasma cells are characterized by a lack of surface membrane immunoglobulin, but have an extensive production and secretion of antibodies of one isotype and specificity for a single epitope (idiotype).

Figure 1-14

Antibody diversity is principally generated by immunoglobulin gene rearrangement. H-chain gene rearrangement is depicted.

Structures

The basic structure of all immunoglobulin molecules consists of two identical L chains and two identical H chains (Fig. 1-15). The antibody molecule consists of three major domains connected by a hinge region. As shown in Fig. 1-15, digestion of antibody molecule with a proteolytic enzyme-papain results in the separation of these three domains. Two domains are identical and are called fragment antigen binding (Fab), and the third domain is called fraction crystallizable (Fc). However, treatment with proteolytic enzyme pepsin results in a fragment that contains both antigen binding arms (Fab')₂ and several pieces of the Fc fragment. Fab interact with the antigen, and Fc bind to Fc-receptors on different cells.

Figure 1-15

Prototypic structure of immunoglobulins. The complementarity regions (e.g., antigen receptor sites that make specific contact with ligands) sites of the V region are shown in the insert.

Various forms of immunoglobulins such as IgG and IgE are found as monomers, secreted IgA as dimers and IgM as pentamers (Fig. 1-16). Consequently, two distinct regions of the assembled immunoglobulin occur: the first, which binds to an antigenic determinant and the second, which has other functions, such as binding to special cells and the first component of complement. The two H chains and each H chain and L chain are linked by disulfide bonds. Each chain is divided into two regions: the C region at the carboxyl-terminus and the variable region at the amino-terminus. The C region of each L chain consists of about 107 amino acids and has an invariant structure except for isotypic features (κ or λ) and allotypic variants (e.g., molecular structures that are individually inherited). V and C regions of H chains are further divided into domains characterized by folding of the polypeptide chain into 110 amino acid loops. V regions of H and L chains display great variability in the sequence of amino acids. Localized areas of these hypervariable regions of H and L chains interact to form antigen binding sites (i.e., CD1, CD2 and CD3; Fig. 1-15). In contrast, C regions of H chains dictate other functions of immunoglobulins, including binding to cell surface receptors. Eight immunoglobulin isotypes, IgG1, IgG2, IgG3, IgG4, IgA1, IgA2, IgM, IgD, and IgE, are produced by B cells as a result of rearrangements of V genes for H chains (VH), D genes for H chains (DH), J genes for H chains (JH), V genes for L chains (VL), J genes for L chains (JL), and C region genes (vide infra) The special properties of each immunoglobulin class are as follows (Table 1-3).

Figure 1-16

Diagram of various forms of immunoglobins; IgG and IgE are found as monomers, secreted IgA as dimers and IgM as pentamers. Dimers and pentamers are held together by the J chain.

Table 1-3

Major Properties of Immunoglobulins.

IgA

IgA consists of α heavy chains and κ or λ light chains. There are two principal molecular forms of IgA, monomers whose basic structure and numbers of domains are similar to IgG and dimers that bind to J chains. Monomeric IgA, the second most common immunoglobulin in adult serum, is primarily produced by plasma cells in the bone marrow, whereas dimeric IgA, the dominant immunoglobulin in external secretions, is produced by plasma cells at mucosal sites.

Dimeric IgA is complexed and transported with a secretory component to form secretory IgA (Fig. 1-17). Dimeric IgA binds to polymeric immunoglobulin receptors (secretory component) on the basolateral membranes of epithelial cells; the complex is internalized and transported across the cells in an endocytic vesicle to the apical pole of the cell where it is secreted as secretory IgA. The addition of a secretory component not only facilitates the transport of dimeric IgA, but protects the molecule from proteolysis.

Figure 1-17

Assembly and secretion of secretory IgA.

There are two subclasses of IgA, IgA1 and IgA2. IgA1 predominates in the blood; there is an equal distribution of the two subclasses in external secretions. IgA2 is more resistant than IgA1 to bacterial IgA proteases that attack the hinge region of the molecule.

Sequence of Antibody Formation

Initial exposure to an antigen results in the production of low affinity antibodies, but continued exposure to antigen leads to the production of high affinity antibodies. In the primary antibody response (the first immunization), B cells are activated to produce IgM antibody. By 3–5 days, specific antibodies, mainly of the IgM isotype, appear in the serum and the concentration (titer) increases until a peak is reached in 10–14 days (Fig. 1-18). Antibody titers then fall to preimmunization levels after some weeks. Upon reimmunization, there is a more rapid and extensive development of antibody-producing cells in regional lymph nodes, and many of them undergo an isotype switch to produce IgG or other immunoglobulin classes of specific antibodies. As a result, in most cases following re-immunization, serum antibodies are primarily IgG and have a greater affinity for antigens; also, the antibody titers are higher and persist for much longer periods.

Figure 1-18

Isotypes of serum antibodies in primary and secondary immunization.

Antibody Binding to Antigen

There are a number of important consequences of antibody binding to antigens, depending upon the nature of the antigen. These include the neutralization of adherence sites or toxins from bacteria, the formation of opsonins, and the activation of the classical pathway of the complement system for that purpose or to create other bioactive factors that enhance inflammatory reactions. 1) In the case of IgM, antigen-antibody complexes are created that most efficiently activate the classical pathway of the complement system and thereby lead to the formation of functional complement fragments including opsonins that facilitate the removal of the complexes by the RES. 2) IgG antibodies, which are the dominant immunoglobulins in extracellular fluids, neutralize toxins and viruses, opsonize particles for ingestion by phagocytes, or when complexed to antigens, activate the classical pathway of complement. 3) Secretory IgA antibodies defend mucosal sites by binding toxins and preventing adhesion of microbial pathogens. 4) IgE antibodies on the surface of mast cells and basophils play an important role in defense against parasites and development of immediate hypersensitivity as previously noted.

Genetic Basis of Antibody Diversity

Specific antibodies are generated as a consequence of immunoglobulin gene rearrangement, i.e., recombination of V, D, J, and C genes (Fig. 1-14). The immune system generates millions of different antibody molecules from the pool of V genes. Separate sets of V genes encode the variable domains of immunoglobulin H and L chains. The two chains are produced separately, but the mechanisms by which their diversity is achieved are similar in principle.

Generation of Antibody Diversity

Antibody diversity is generated by the following mechanisms.

Generation of TcR Diversity

The combinatorial diversity of TcR is greatly increased by junctional diversity, i.e., the variability of the junctions between different VDJ genes. New nucleotide base pairs are often added at the junction. Indeed, the junctional diversity of TcR is several orders of magnitude greater than that of an immunoglobulin gene. On the other hand, rearranged TcR genes are not subject to somatic mutations that contribute significantly to the generation of antibody diversity. The lack of somatic mutations appears to be related to the fact that the α/β T cells always recognize a complex of antigenic fragment with the self MHC molecule. The receptor mutation could divert the specificity towards self molecules.

General Features

These molecules play a very important role in the recognition of self and non-self antigens by T cells. The MHC consists of a cluster of >100 genes on chromosome 6 that encode a number of biologically important molecules (Fig. 1-9). These molecules are responsible for the rejection of tissue grafts by genetically disparate individuals, as the name histocompatibility indicates. These molecules present antigens to T lymphocytes; govern interactions between T cells, B cells and accessory cells; and control the intrathymic development of the TcR repertoire against foreign antigens (positive selection) and against self (negative selection) Human MHC protein products are called human leukocyte antigens (HLA).

Genes and Structures

The two most important HLA glycoproteins are designated as class I and class II molecules (Fig. 1-9).

MHC Class I/II Molecules

MHC class I molecules are ubiquitous on somatic cells whereas MHC class II molecules are restricted to monocytes, macrophages, dendritic cells, B cells, Langerhans cells, keratinocytes, activated T cells and certain types of epithelial cells. MHC class I molecules have three extracellular domains (α1, α2 and β1), and a cytoplasmic tail. In contrast, MHC class II molecules have four extracellular domains (α1, α2, β1 and β2).

Three genes encode three independently expressed MHC class I molecules: HLA-A, -B and -C. Each gene contains three exons for the domains 1, 2 and 3. The MHC class II cluster, HLA-D, also contains three distinct genes, DP, DQ and DR, each of which has a separate set of exons for the α and β chain.

MHC Alleles

An important aspect of the HLA gene system is its polymorphism. Each gene, MHC class I (A, B and C) and MHC class II (DP, DQ and DR) exists in different forms, or alleles. HLA alleles are designated by numbers and subscripts. For example, two unrelated individuals may carry class I HLA-B, genes B5, and Bw41, respectively. Allelic gene products differ in one or more amino acids in the α and/or β domain(s). Large panels of specific antibodies are used to type HLA haplotypes of individuals using leukocytes that express class I and class II molecules. HLA typing is used for matching donors and recipients for organ/tissue transplantation and to predict the risk of certain diseases . In addition, the polymorphism of HLA genes has major implications for the function of class I and class II molecules (vide infra).

Role in Antigen Presentation

MHC molecules are required for antigen presentation to T cells (Fig. 1-12). Peptides associated with MHC class I and class II molecules are recognized by CD8⁺ and CD4⁺ T cells, respectively. Foreign protein antigens are taken up by various types of cells in the body, internalized and subjected to enzymatic degradation called antigen processing. Antigenic peptide fragments bind to MHC class II molecules and are then transported to the cell surface. This MHC/antigen complex is recognized by the TcR on CD4⁺ T cells. A CD4⁺ T cell activated by an appropriate class II/peptide antigen complex on an antigen-presenting cell, such as a B cell or macrophage, may become a helper cell for antibody or cell-mediated immune responses.

A different scenario is found for viral antigens in infected cells or tumor antigens. These antigens are processed to fragments, which are expressed in association with the class I molecule. The MHC class I/antigen complex is recognized via the TcR by CD8⁺ T lymphocytes, which become activated and differentiate into CTLs that destroy infected cells.

Variable domains of MHC class II molecules encoded by some allelic genes may be unable to bind a given antigenic peptide and thus fail to present the peptide to antigen-specific T cells. As a result, an immune response to this antigen cannot be mounted. Because of the association of high and low responses to a specific antigen with particular MHC class II alleles, MHC genes have been termed immune response genes. HLA typing reveals that individuals carrying certain alleles are at a higher risk of developing diseases such as ankylosing spondylitis, myasthenia gravis or type I diabetes mellitus. It is likely that this association reflects an underlying immunopathologic reaction involving MHC class I/II molecules or an association with other genes in the MHC.

MHC products control the selection of the immune repertoire of T lymphocytes. T cells interact with MHC class I/II molecules during their maturation in the thymus. This interaction kills immature cells whose TcR have a high affinity for self-MHC or for an MHC/self-protein complex by a mechanism called apoptosis or programmed cell death. Potentially autoreactive T cells would be eliminated in this fashion (negative selection). Furthermore, the selection process by MHC molecules determines the T cell repertoire of the individual against various foreign antigens (positive selection). This negative and positive sorting of T cells is called thymic selection.

T Cell Activation

Although, the presentation of antigen in the context of MHC molecules is essential for T cell recognition of peptide antigens, interactions between the MHC-bound peptide and TcR and the MHC class I or class II molecules, respectively, with CD8 or CD4 is not sufficient to activate T cells. Other ligands on antigen-presenting cells and their receptors on T cells are required to complete the process. These ligand-receptor interactions include the ligands ICAM-1, LFA-3, and B7-1/2 on antigen-presenting cells binding to their receptors LFA-1, CD2, and CD28/CTLA-4, respectively, on T cells (Fig. 1-19). These and other counterstructures for B7-1 and B7-2 appear to precisely control the extent of T cell activation.

Figure 1-19

Ligand-receptor interaction necessary for optimal T-cell activation. The requirements for CD8⁺ T cells are the same except for interactions between MHC class I molecules and CD8 molecules.

X-Linked Agammaglobulinemia

In this immunoglobulin deficiency disease, there is a genetic defect in the development of B cells from pre-B cells in the bone marrow. The defect is due to mutations in the gene that encodes for B cell tyrosine kinase. Consequently, the B cells are not produced. Because of the block in the development of B cells, germinal centers, plasma cells, and specific antibodies are profoundly reduced. The rest of the immune system is normal. Affected individuals are unusually susceptible to infection by virulent encapsulated respiratory bacteria and enteroviruses. These patients benefit greatly from intravenous infusions of human IgG.

Hyper-IGM Antibody Deficiency

A second X-linked defect in antibody formation, the hyper-IgM antibody deficiency, is characterized by a block in immunoglobulin class switching. Consequently, IgM (and IgD) antibodies are produced, but IgA and IgG antibodies are not. These patients are also unusually susceptible to infection by virulent encapsulated respiratory bacteria. The disease is due to mutation in the gene that encodes for CD40 ligand (CD39) on T cells. Because of the defect, T and B cell interactions are insufficient for immunoglobulin class switching. These patients also benefit greatly from intravenous infusions of human IgG.

Severe Combined Immunodeficiency (SCID)

The most common type of SCID is due to stop codon defects in the X-chromosome gene that encodes for the γ-chain that is common to IL-2, IL-4, IL-7, IL-9, and IL-15 receptors. A more moderate combined immunodeficiency disease has been reported and is due to a missense point mutation in a part of the gene that encodes for the cytoplasmic region of that gene. In addition, other defects reported to cause SCID involve an autosomal recessive defect in the formation of adenosine deaminase, a defect in the formation of CD3, a defect in the post TcR-CD3 receptors' signalling, and deficiency in the formation of IL-2.

Patients with these diseases display few T lymphocytes, decreased T cell functions, poor antibody formation, and variable numbers of B cells and serum concentrations of immunoglobulins. As a consequence of the deficiencies in T cells, these patients are very susceptible to opportunistic pathogens, including Candida albicans, Salmonellae, Pneumocystis carinii, Cytomegalovirus, and Varicella zoster virus. Patients with SCID usually die before the age of two years, unless definitive immunologic interventions are instituted.

Specific treatments have been devised for patients with SCID. Many patients have been treated successfully with bone marrow transplants to supply normal stem cells. Patients with adenosine deaminase deficiency may also be managed by infusing the enzyme packaged with polyethylene glycol. Recently, some patients with adenosine deaminase deficiency have been successfully treated with gene therapy. Although the beneficial effects have not been permanent, nevertheless, they are encouraging.

Two major intrinsic defects in the function of phagocytic cells have been recognized. The first one is an autosomal recessive defect in the formation of the common β-subunit of the family of adherence glycoproteins (integrins). The deficiency interferes with the ability of these leukocytes to adhere to the surface of endothelial cells. Consequently, the motility of these cells on two-dimensional surfaces is impaired. Thus, this defect results in bacterial infections in interstitial sites, such as in the skin and periodontium.

The second disorder, chronic granulomatous disease, was the first recognized genetic defect of the function of phagocytic cells. The disease is X-linked in ~ 70% of affected patients. In those cases, the gene for the gp90 protein subunit of cytochrome b₅₅₈ is abnormal. Consequently, the protein is not produced, the cytochrome does not persist, and intracellular killing is impaired. Less frequently (~ 3% of cases), the disease is due to a defect in the autosomal gene for the lower molecular weight subunit of the heterodimer, p22^phox. Autosomal defects in genes for cytoplasmic proteins that stabilize the cytochrome have also been recognized.

In these disorders, phagocytes are unable to mount a respiratory burst and therefore are unable to produce toxic oxygen compounds, such as hydrogen peroxide, which are required for intracellular killing of catalase-positive microorganisms such as Candida albicans, Escherichia coli, and Serratia species. The failure to kill catalase-positive microorganisms occurs because the microbial agents do not supply the oxygen substrates required for intracellular killing. In contrast, these dysfunctional neutrophils kill catalase-negative microorganisms such as the streptococci since those microorganisms bring hydrogen peroxide into the phagolysosome.

Protein-Calorie Malnutrition

Protein-calorie malnutrition is the leading cause of immune deficiency in the world. Protein-calorie malnutrition leads principally to a profound deficiency in the production and function of T cells, rendering the victim susceptible to many of the opportunistic infections that occur in genetic T cell deficiencies. With increasing protein-energy deficiency other parts of the immune system are also affected. Specific nutrient deficits such as iron or vitamin A deficiency also depress certain parts of the immune system.

Certain types of infections temporarily depress parts of the immune system. For example, many acute viral infections suppress cellular immunity for several days to a few weeks, and serious bacterial infections inhibit the ability of neutrophils to respond to chemotactic agents. Furthermore in schistosomiasis, Th1 responses are accentuated and Th2 responses are suppressed. This leads to a decreased ability to form antibodies after antigenic challenges.

Malnutrition and infection interact to inhibit the immune system. As a result of malnutrition, the immune system becomes compromised. That leads to respiratory and gastrointestinal infections. Those infections may in turn further interfere with the immune system. Moreover, some infections further compromise nutritional status by increasing nutrient losses (intestinal malabsorption) and utilization (fever, caloric expenditure during sweating) or by interfering with normal nutrient metabolic pathways (cachectic effect of TNF-α). Consequently, immune function is further impaired.

Human Immunodeficiency Virus Infections

A second acquired immunodeficiency is due to human immunodeficiency virus (HIV) infection. This retrovirus infection was first encountered in homosexual males and individuals who were injecting illicit drugs or who received blood products contaminated with the virus. The infection has since spread to heterosexual populations by sexual transmission. The infection has reached epidemic proportions in developed as well as developing countries and continues to increase. The resultant acquired immune deficiency syndrome (AIDS) occurs because the virus infects and destroys CD4⁺ T cells. The virus binds to the CD4 surface antigen on T cells and to the same or similar moiety on macrophages. Since CD4⁺ T cells are essential for the genesis of cellular immunity and for orchestrating the function of many other parts of the immune system, a deficiency in these T cells increases the patient's susceptibility to opportunistic infections. The vast majority of such infected patients die after several years. No preventative immunizations or curative treatments are available for the infection at this time.