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
Box 4-1.
Are You a Human or a Mouse?
This archaic challenge, issued to someone showing classic symptoms of soft spine syndrome, in the hopes of getting that person to act decisively and courageously, actually applies to antibodies as well. Although humans and mice are much more closely related than many of us would like to admit, there are subtle but real differences between some antibody classes shared by humans and mice. Accordingly, there is also a somewhat different nomenclature used to designate these antibody classes. These different designations can be confusing, especially in the case of the IgG subtypes. A guide to the different antibody designations in humans and mice is provided below:
Why such differences occur between closely related species remains a fascinating but unanswered question.
IgG1 and IgG3, but not IgG2 and IgG4, strongly activate the classical pathway of the complement cascade. The six head domains of the C1q complex bind to the Fc region of IgG monomers bound to the surface of the antigen (see Figure 3-14). This cross-linking activates the C1 complex and initiates the complement cascade. IgG1 and IgG3 are also called opsonizing antibodies because these two subtypes are the most effective in opsonizing microbes due to the fact that they have the highest affinity to Fc receptors on phagocytes (see Figure 3-12). IgG2 and IgG4 opsonize poorly, if at all. IgG2 appears to be best at recognizing carbohydrate or polysaccharide-containing antigens. IgG4 appears to play a regulatory role by dampening Fc receptor-mediated inflammatory responses and is produced when persistent exposure to a particular antigen occurs.
Normally, antibody binding to an antigen on the surface of an extracellular pathogen facilitates ingestion of the opsonized antigen by phagocytic cells or triggers complement-dependent killing at the cell surface (see chapter 3). However, in the case of intracellular infections of tissues, IgG (mostly IgG1 and IgG3, but to some extent also IgG2, as well as IgA and IgE) can also mediate a different response, called antibody-dependent cell-mediated cytotoxicity (ADCC). In ADCC, antigens presented on the surfaces of infected cells bind to the Fab regions of specific IgG molecules. The exposed Fc portions of the IgG molecules are then free to bind to effector cells (PMNs, macrophages, or NK cells) that contain Fc receptors on their surfaces. In the case of NK cells, this linkage triggers a cytotoxic bombardment of the infected cell, as described in chapter 3. In the case of macrophages and PMNs a similar killing response can also be elicited, instead of phagocytosis. Thus, by linking together players of the innate (NK cells, PMNs, and macrophages) and adaptive (antibodies) immune responses, ADCC serves as an important defense against intracellular pathogens (more on this later).
IgM. IgM consists of a complex of five or more monomers that are connected via disulfide bonds to each other and to a peptide called the J-chain (Figure 4-1). IgM is found mostly in serum, where it accounts for 5 to 10% of the total serum Ig, but it is also secreted at mucosal surfaces and in breast milk. IgM is the most effective activator of complement. Because of its polymeric form, a single molecule of IgM has five Fc regions that can be complexed, which is sufficient to bind C1q and strongly activate complement via the classical pathway or to bind and cluster multiple Fc receptors and mediate enhanced opsonization.
IgM is the first antibody type expressed in the fetus (at 20 weeks’ gestation). During B cell differentiation, IgM is the only antibody expressed by naïve B cells. Once B cells mature and proliferate, they start to produce IgG (or IgA) and IgM antibody expression wanes. IgM predominates in the initial (primary) antibody response against a pathogenic microbe, whereas IgG (or IgA at mucosal sites) predominates in response to sustained or subsequent infections (secondary antibody response) by the same microbe (Figure 4-3). Because IgG can circulate in serum for long durations, detectable IgG levels may indicate the presence of a current infection that is well underway or may simply be the residue of a previous infection, whereas IgM levels are detectable during an initial infection but vanish once production of IgG antibodies begins. This aspect of the immune response is actually exploited by diagnostic labs to determine whether a patient is experiencing a first exposure (acute infection) to a particular microbe.
