Antibody

ANTIBODY

The basic structure of the antibody molecule is depicted in Figures 1.2A and B. 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 anti-gen-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 dif-ferences 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 anti-body 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 (C_H). A C_l chain may consist of either

Figure 1.2A Heavy and light chains of an IgG antibody. An IgM antibody would be a pentameric structure of an IgG molecule.

two kappa (κ) or two lambda (λ) chains but never one of each. Of all the human anti-body molecules, approximately 60%, are κ chains and 40% contain λ chains. Although there are no known differences in the func-tional properties of κ and λ chains, there are several different types of the C_H domain. These differences are reflected in determin-ing the class (isotype) of the antibody and thereby the physiological function of a par-ticular antibody molecule.

The IgM molecule is the oldest class of immunoglobulins, and it is a large mol-ecule consisting of five 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 five 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 affinity binding to the antigen in question. Second, because of its size, it does not usu-ally 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 efficiently and clear most protein antigens, including the removal of microorganisms by phago-cytic 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 pro-vide immune protection to the neonate.

Figure 1.2B Antigen-binding sites and antigen binding in an IgG antibody. Hinge region allows for rotational and lateral movements of the two antigen-binding sites.

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 anti-gens 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 specific IgE recep-tors on most cells and basophiles. This molecule  plays an extremely

important role in allergic reactions and expelling intestinal parasites, which is accomplished by increasing vascular per-meability and inducing chemotactive fac-tors following mast cell degranulation

Given this extraordinary ability to gen-erate 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

Figure 1.3 The genetics of antibody production

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 V_H domain, is then joined at the level of RNA processing to C_u and the B cell makes an IgM molecule. By omitting the C_u gene and joining V_HD_J to a C_λ an IgG molecule is produced. This enormous versatility allows the cell to make IgM, IgD, IgG, IgA, or Ig 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 immunodeficiency status occurs in these animals, characterized by absent mature B and T cells.

Thus, the diversity of antigen bind-ing is achieved by the large number of V

Figure 1.4 Recombination events necessary for generation of class and subclass switching.

genes available and their combination with different D and L genes to provide differ-ent antibodies. Furthermore, the inherited set of genes may be increased by somatic mutation during multiple divisions of lym-phoid cells, thereby increasing the number of antibody specificities to 10¹⁴, which far exceeds the number of B cells (10¹⁰) in the body.

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