The genetics of antibody production

The major mucosal immunoglobulin, IgA, consists of two basic units joined by a J chain. The addition of a secretion molecule prevents its digestion by enzymes present in mucosal and intestinal secretions. Thus, IgA2 is the major IgA molecule in secretions and is quite effective in neutralizing antigens that enter via these mucosal routes. IgA1, the main IgA molecule in serum, is, however, susceptible to inactivation by serum proteases and is thus less active for defense. Its function is unclear at present. 

Two other classes are worthy of note. IgD is synthesized by antigen-sensitive B cells and is involved in the activation of these cells by antigen. IgE is produced by plasma cells and binds to specifi c IgE receptors on most cells and basophiles. This molecule (see Chapter 9) plays an extremely important role in allergic reactions and expelling intestinal parasites, which is accomplished by increasing vascular permeability and inducing chemotactive factors following mast cell degranulation. 

Given this extraordinary ability to generate large numbers of antibody molecules, how does the immune system recognize all pathogens, including past, present, and future? This diversity is achieved by the way in which the genetics of antibody production is arranged (see Figure 1.3). The light and heavy chains are carried on different chromosomes. The heavy chain genes are carried on chromosome 14. These genes are broken up into coding systems called exons with intervening segments of silent segments called entrons. The exons represent the central region of the heavy chain and a large number of V regions. Between the V and D genes are two small sets of exons called the D and J. With each single B cell, one V gene is joined to one D and J in the chromosome. The product, the VH domain, is then joined at the level of RNA processing to Cu and the B cell makes an IgM molecule. By omitting the Cu gene and joining VHDJ to a Cλ an IgG molecule is produced. This enormous versatility allows the cell to make IgM, IgD, IgG, IgA, or IgE in sequence while using the same variable regions (see Figure 1.4). The heavy chain gene recombinations are controlled by two recombination activity genes called RAG1 and RAG2. If these genes are eliminated by “knock-out” techniques in mice, profound immunodefi ciency status occurs in these animals, characterized by absent mature B and T cells. 

Thus, the diversity of antigen binding is achieved by the large number of V genes available and their combination with different D and L genes to provide different antibodies. Furthermore, the inherited set of genes may be increased by somatic mutation during multiple divisions of lymphoid cells, thereby increasing the number of antibody specifi cities to 1014, which far exceeds the number of B cells (1010) in the body. 

Once a given B cell is preselected to produce a particular VH and VL domain, all the ensuing progeny of that B cell will produce the same VH or VL domain. The sequence of events is as follows: initially, the B cell produces intracellular antigen-specific IgM, which becomes bound to the cell surface. The B cell is now antigen responsive with exposure to a given antigen. The committed B cell begins producing a certain isotype or class of immunoglobulins and begins dividing, and all the progeny will produce the identical immunoglobulin molecules. These B cells will later mature into either plasma cells or long-term memory B cells.

ANTIGENS

Antigens are any substances that are capable, under appropriate conditions, of inducing the formation of antibodies and reacting specifi cally with the antibodies so produced. They react with both T-cell recognition receptors and with antibodies. These antigenic molecules may have several antigenic determinants, called epitopes, and each epitope can bind with a specifi c antibody. Thus, a single antigen can bind to many different antibodies with different binding sites.

Some low-molecular-weight molecules called haptens are unable to evoke an immune response but can react with existing antibodies. These molecules need to be coupled to a carrier molecule to be antigenic. 

For some molecules such as drugs, the molecule needs to be conjugated to a carrier. The carrier may be a host protein. The tertiary structure of the molecule as well as the amino acid sequence is important in determining antigenicity. Certain structures such as lipids and DNA are generally poor antigens. 

Most antigens are either thymusdependent or thymus-independent antigens. Thymus-dependent antigens require T-cell participation: Most proteins and foreign red cells are examples of these molecules. Thymus-independent antigens do not require T-cell participation for antibody production. Instead, they directly stimulate specifi c B lymphocytes by crosslinking antigen receptors on the surface of B cells. These molecules produce primarily IgM and IgG2 antibodies and do not stimulate long-lasting memory cells. Most bacterial polysaccharides (found in bacterial cell walls) fall into this category. Certain polysaccharides, such as LPS (lipopolysaccharide), not only induce specifi c B-cell activation but also can act as a polyclonal B-cell stimulant.

ANTIBODY

It consists of a four-chain structure divided into two identical heavy (H) chains with a molecular weight of 25 kDa. Each chain is composed of domains of 110 amino acids and is connected in a loop by a disulfide bond between two cysteine residues in the chain. 

The amino acid N-terminal domains of the heavy and light chains include the antigen-binding site. The amino acids of these variable domains vary between different antibody molecules and are thus known as the variable (V) regions. Most of these differences reside in the hypervariable areas of the molecule and are usually only six to ten amino acid residues in length. When the hypervariable regions in each chain come together along with the counterparts on the other pair of H and L chains, they form the antigen-binding site. This part of the molecule is unique to the molecule and is known as the idiotype determinant. In any individual, 106 to 107 different antibody molecules can be composed from 103 different heavy and light chains of the variable regions. The part of the molecule next to the V region is called the constant (C) region made up of one domain in the light chain (C1) and three or four in a heavy chain (CH). A Cl chain may consist of either two kappa (κ) or two lambda (λ) chains but never one of each. Of all the human antibody molecules, approximately 60%, are κ chains and 40% contain λ chains. Although there are no known differences in the functional properties of κ and λ chains, there are several different types of the CH domain. These differences are refl ected in determining the class (isotype) of the antibody and thereby the physiological function of a particular antibody molecule.

The IgM molecule is the oldest class of immunoglobulins, and it is a large molecule consisting of fi ve basic units held together by a J chain. The major role IgM plays is the intravascular neutralization of organisms, especially viruses. The reason for this important physiological role is that it contains fi ve complement-binding sites, resulting in excellent complement activation. This activation permits the segment removal of antigen–antibody complement complexes via complement receptors on phagocytic cells or complement-mediated lysis of the organism. However, in contrast to the IgG molecule, it has relatively low affi nity binding to the antigen in question. Second, because of its size, it does not usually penetrate into tissues. In contrast, IgG is a smaller molecule that penetrates easily into tissues. There are four major classes of IgG: IgG1 and IgG3 activate complement effi ciently and clear most protein antigens, including the removal of microorganisms by phagocytic cells. 

In contrast, IgG2 and IgG4 react mostly with carbohydrate antigens and are relatively poor opsonins. This is the only molecule that crosses the placenta to provide immune protection to the neonate.