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.
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).
There are four subclasses of IgG, each of which displays unique antigenic determinants on the C region of the H chains. The approximate proportion of each subclass in blood is IgG1, 70%; IgG2, 20%; IgG3, 8%; and IgG4, 2%. The antibody specificities are distributed in somewhat specific patterns in each subclass. Neutralizing antibodies to protein toxins are mostly found in IgG1, antibodies to polysaccharides in IgG2, and antibodies to viruses in IgG3.
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.
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.
Figure 1-15
The complementarity regions (e.g., antigen receptor sites that make specific contact with ligands) sites of the V region are shown in the insert.
Figure 1-16
Dimers and pentamers are held together by the J chain.
Table 1-3
IgG
IgG is a monomeric, four-chain structure consisting of two γ heavy chains and two κ or λ light chains. The C region of the H chain of the molecule consists of three domains. Inter-chain disulfide linkages between the Cγ1 and Cγ2 domains stabilize the structure and define the hinge region of the molecule. IgG is the dominant immunoglobulin in extracellular fluids and is the only immunoglobulin transported across the placenta, and directly acts as an opsonin.There are four subclasses of IgG, each of which displays unique antigenic determinants on the C region of the H chains. The approximate proportion of each subclass in blood is IgG1, 70%; IgG2, 20%; IgG3, 8%; and IgG4, 2%. The antibody specificities are distributed in somewhat specific patterns in each subclass. Neutralizing antibodies to protein toxins are mostly found in IgG1, antibodies to polysaccharides in IgG2, and antibodies to viruses in IgG3.
IgM
IgM is a pentamer of 4-chain units that are bound to a separate peptide called the J chain. IgM molecules consist of μ H chains and κ or λ L chains. Monomeric IgM is the principal antigen receptor on B cells. IgM is found principally in blood, but also occurs in external secretions. It binds most efficiently the C1q subunit of the first component of complement (vide infra), and is the first immunoglobulin expressed in B cell development.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
IgD
IgD is a monomeric four-chain polypeptide structure that is similar to IgG but its heavy chain (δ) is unique. Although this protein is expressed along with monomeric IgM on mature B cells, only small amounts of it are found in extracellular fluids.IgE
IgE is also a four-chain polypeptide structure that is similar to IgG, but its heavy chain (ε) is distinct. Only trace amounts of this immunoglobulin are found in serum. IgE binds avidly to circulating blood basophils and mast cells in the submucosal sites and the skin. Cell-bound IgE antibodies defend against tissue parasites and initiate the pathogenesis of immediate hypersensitivity by triggering the release of low-molecular weight vasoactive compounds, including histamine, leukotrienes, and platelet-activating factor and certain proinflammatory cytokines such as TNF-α and IL-5, once they are cross-linked by antigens.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
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.
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.