antibodies perform such diverse tasks, it is first necessary to understand how each type of antibody is put together and how they work.
Characteristics of Antibodies and Their Diverse Roles in Preventing Infection
The basic structures of the five types of antibodies (IgG, IgM, IgD, IgE, and IgA/sIgA) are shown in Figure 4-1. The antibody monomer consists of a complex of two heavy chains (H-chains) and two light chains (L-chains). The words “heavy” and “light” refer to the size of the peptide chain, with the H-chain being the larger of the two. The H-chains and L-chains are held together by a combination of disulfide bonds and noncovalent interactions. The H-chains define the type of antibody, with the H-chains for IgE and IgM being longer than those for IgA, IgD, and IgG.
Figure 4-1. Structures of IgG, sIgA, IgM, IgD, and IgE antibodies. Each antibody monomer is a complex comprised of four peptide chains: two identical, larger heavy chains (H-chains) and two identical, smaller light chains (L -chains) that are covalently attached through disulfide bonds. The type of H-chain defines the antibody class, with the H-chains for IgE and IgM being longer than those of IgA, IgD, and IgG. IgG is the major class of circulating antibodies. Each monomer recognizes the target epitope via two antigen-binding sites that are located in the variable regions (Fv) of the Fab portion of the monomer. There are four IgG subtypes (IgG1, IgG2, IgG3, and IgG4). The Fc region of the molecule is responsible for complement activation through binding C1q and for enhanced opsonization through binding to phagocyte Fc receptors. IgA has two subtypes: IgA1, which is found mostly in serum, and IgA2, which forms a dimer of two IgA monomers linked via a polypeptide joining chain (J-chain) and is secreted at mucosal surfaces. The IgA dimer acquires a secretory piece during transport through mucosal epithelial cells and is released as secretory IgA (sIgA) into the lumen, where it binds to mucin. IgD is a monomer that is expressed on the surface of mature B cells or is secreted. IgM is a multimer (mostly pentamer) of IgM molecules linked via disulfide bonds and a J-chain. IgM is highly agglutinating (binds and clumps antigens) and strongly activates complement (thousandfold better than IgG). IgE binds to IgE receptors on mast cells and basophils that, upon binding of antigen, trigger release of histamine and inflammatory cytokines.
Antibodies have two important regions: an N-terminal antigen-binding region (Fab) that harbors the end of the antibody that binds to a substance considered foreign by the body and a C-terminal constant region (Fc) that confers host specificity and interacts with host cells. In the most common antibody, IgG, the Fc region interacts with complement component C1q via a glycosylated region or with phagocytic cells via an Fc receptor-binding region. The Fab region contains a constant region and a variable region (Fv) that binds to a specific antigen. An antigen is defined as any material the body recognizes as foreign (nonself) that binds to an antibody. An antigen that binds to an antibody molecule can be an infectious microbe or some protein, nucleic acid, or carbohydrate component of the microbe. Additional examples of types of antigens are proteins, macromolecules, or organs from noncompatible human donors or other animals, as well as molecules from pets, plants, or insects, including dander, pollens, and toxins.
The humoral adaptive immune system’s ability to recognize a wide range of antigens and subsequently adapt to new invading pathogens relies on the ability of the B cell population to produce a vast array of antibodies with enormous diversity. This vast diversity of antigen specificity is possible because the immunoglobulin (Ig) genes undergo many gene recombination, rearrangement, insertion, deletion, and splicing events from separate, different gene segments encoding different regions of the antibody molecule during B cell development. A detailed description of the mechanism that generates the large, highly diverse population of B cells, each producing a different specific antibody that can bind to the different kinds of antigens, is beyond the scope of this book. But briefly, an amazing DNA recombination process, called V(D)J recombination, randomly shuffles a wide repertoire of variable (V), diversity (D), and joining (J) gene segments that form the N-terminal Fab regions of H-chain proteins (or just V and J gene segments in the case of L-chain proteins) and fuses them to gene segments corresponding to the constant regions in the Ig genes during the development of B cells. The exact pattern of rearrangements occurs independently in each B cell. RNA transcripts of the resulting mosaic Ig genes are further processed to give expression of the specific antibody produced by each B cell in the population. Normally during this developmental process, any B cells that produce Ig molecules that recognize self-antigens are eliminated. All of the antibodies produced by any given B cell have identical antigen-binding sites. But, during clonal expansion, when an antigen and Th cells stimulate a particular subset of B cells, a fairly high rate of spontaneous mutations, termed somatic hypermutation, is introduced, which serves to increase the diversity of the antibody pool even more and leads to proliferation of B cells that produce antibodies with high affinity to their cognate antigens.
An antibody monomer has two antigen-binding sites, each of which recognizes and binds to the same specific segment of an antigen. The antigen-binding sites are grooves in the antibody Fv ends that only bind tightly to a molecule having one particular structure, called an epitope or antigenic determinant. An epitope on a protein antigen can vary in size from 4 to 16 amino acids, although most are 5 to 8 amino acids in length. Complex antigens such as microbes contain many possible epitopes, each binding to a different antibody. Epitopes can be continuous or discontinuous, based on whether the antibody recognizes the primary (linear peptide) or tertiary (topographical) structures of the protein, respectively (Figure 4-2).
Figure 4-2. Continuous versus discontinuous epitopes. Epitopes of protein antigens can be continuous based on the linear amino acid sequence (primary structure) of the protein or discontinuous based on the tertiary conformational structure of the protein.
An immunogen is an antigen that elicits an immune response, but it is important to note that not all antigens are immunogens. In practice, only a subset of the epitopes on an antigen dominates the specific response to that antigen. Why some epitopes are highly immunogenic (i.e., elicit a robust antibody or T cell response) while others are only weakly immunogenic is still not well understood. Immunogenicity is often based on the size and complexity of the antigen molecule and is reflected in the antigenicity of different types of macromolecules. In general, proteins are better immunogens than carbohydrates, which are in turn better immunogens than nucleic acids and lipids.
Serum Antibodies
IgG. IgG, produced as a monomer, is the most prevalent type of antibody in blood and extravascular fluid spaces (approximately 80% of circulating antibodies are IgG). IgG is the only antibody type that can cross the placenta (via transcytosis bound to the neonatal Fc receptor, FcRn) and is responsible for protecting an infant during the first six months of life until the infant’s adaptive defenses are developed.
There are four different subtypes of IgG antibodies in humans (Table 4-1), named in the order of their abundance in serum: IgG1 (66%), IgG2 (23%), IgG3 (7%), and IgG4 (4%). These subtypes differ not only in their function, but also in their amino acid sequences, glycosylation (posttranslational decoration of the IgG protein by sugars), length and flexibility of the hinge region, extent of disulfide bond cross-linking of their H-chains (primarily in the Fc portion), and the specificity of Fc receptor interactions. [Warning: The nomenclature used to describe human IgG is not the same as that used to describe murine IgG. So, IgG1 of mice does not necessarily have the same features as IgG1 of humans. We mention this issue because it explains why different papers on the development of the immune response seem to contradict each other, but actually do not. For a comparative summary of the different types of Ig molecules of humans and mice, see Box 4-1.]
Table 4-1. Protective roles of serum antibodies IgG and IgM