Antibodies are produced by B cells in response to infection. Antibodies form a family of plasma proteins known as immunoglobulins. Antibodies perform two main functions in host defence. They bind to molecules from the pathogen that elicited the immune response and they also recruit other cells and molecules to destroy the pathogen once the antibodies are bound to it. The tertiary structure of an antibody has a “Y” shaped conformation and comprises of two parts: the variable region and the constant region (Figure 1.1). The antigen binding region is known as the variable region or “V” region . The variable regions contain short segments of hypervariable segments. The variability of the antibody molecule allows each molecule to recognize a particular antigen and the vast repertoire of antibodies made by an individual is sufficient to ensure that virtually any structure can be bound. The amino acid composition of the variable region exhibits great diversity facilitating recognition of a wide variety of antigens. The constant region or the “C” region of the antibody directs immunoglobulin effector functions that mediate killing and removal of invading organisms. The constant region shows less diversity and can be subdivided into distinct classes known as isotypes.
Specific markers found in the hypervariable region of any antibody and which are associated with the antigen binding site or the paratope are known as idiotypes. The smallest definable part of an idiotype is an idiotope. Idiotypes are also antigenic. Idiotypes are classified into two groups depending on how they
are recognized by the anti-idotypic antibodies. Private or restricted idiotypes are defined by anti-idiotypic antibodies that react only with the immunizing immunoglobulin. Public or crossreactive idiotypes may be present on antibodies with similar or different antigenic specificities. Public or crossreactive idiotypes may also show common amino acid sequences within the framework regions or complementarity determining regions. The broad implications of idiotype recognition in immunology were propounded by Jeme in 1974. He postulated that the ability of antibodies to both recognize and be recognized creates an interacting network, capable of regulating the immune system. The antibody which recognizes antigen is termed antibody 1 (Abl) to denote the first member of the series. Antibodies which specifically bind to the antigen recognition site of the Abl are called Ab2 antibodies (the second member of the series). The antigenic determinants on Abl which are recognized by Ab2 are called idiotypes. Hence, Ab2 is referred to as anti-idiotype (anti-id) antibodies which in turn trigger the production of Ab3 etc. Thus the entire immune system is devoted to recognizing and regulating idiotopes (Jeme 1984, Klinman 1986). Anti-idiotypes which recognize idiotypes distinct from the antigen binding site and bind to Abl in an antigen non inhibitable fashion are designated as Ab2 alpha, whereas antigen inhibitable anti-id antibodies include Ab2 beta and Ab2 gamma. Anti-id antibodies that bind to paratopes and bear the internal image of the antigen recognized by Abl are called Ab2 beta. This subclass of anti-id antibodies are known to exhibit the biological functions of the primary antigen. Anti-id antibodies that recognize idiotopes in or near the antigen binding region of the Abl but do not mimic the antigen are called Ab2 gamma (Fung et al 1990).
The “Fab” (fragment antigen binding) constitutes the antigen binding region which consists of the variable regions of the light and heavy chains and includes the first heavy chain constant region (C h I ) and the light chain constant region domain (C l). The “Fc” (Fragment crystalline) is a part of the immunoglobulin formed from the constant domains (C h 2 and C h 3) of heavy chain. The “Hinge”region of the antibody gives flexibility to the antibody molecule allowing ease of binding to antigen and cell surface receptors.
Allotypic variation is a phenomenon describing the occurrence of different alleles of the same immunoglobulin between individuals. Such genetic markers are associated with the constant region of immunoglobulins and include the Gm specificities of IgG of which about 25 variants have been identified.
The basic units of an immunoglobulin are the light (L) chain polypeptide with a molecular weight of about 25 kilodaltons (kDa) and the heavy (H) chain polypeptide with a molecular weight of approximately 50-65 kDa. An inununoglobulin monomer consists of two identical L chain molecules covalently linked by disulphide bonds to two identical H chain molecules. The immunoglobulin molecule can be divided into functional components using proteolysis. Cleavage with papain generates two identical fragments called the Fab that retains the ability to bind antigen. Cleavage with pepsin generates a molecule that contains two disulphide-bonded Fab fragments called F(ab)2. Two
Fab regions in a single monomer immunoglobulin provides a bivalent binding pocket for recognition of repititive determinants present in multivalent antigens or a single determinant present on two separate antigen molecules. Within the variable region of the immunoglobulin molecule are discrete regions known as
complementarity determining regions (CDR) that make direct contact with antigen. Amino acid sequences of CDRs are highly variable and are flanked by more conserved aminoacid sequences called framework regions (FWR). The L and H chain molecules contain three CDRs and four FWRs. The antigenic determinant recognized by the heavy and light chain CDRs is called an epitope.
There are two distinct light chain polypeptides called kappa (k) and
lambda (%). Light chains contain a variable region and a single constant region domain. Amino acid residues in the light chain variable region combine the residues from the heavy chain variable region to create a antigen binding region. The kappa light chain is utilized more often (65%) than lambda in humans (Jane way 1997a).
Figure 1.1
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Im m unoçlobulin flçG)
v a r i a b l e r é g io n s c o n s ta n t r e g io n s h in g e CN ro5
KEY: Ca rb ohy d r a teCom plem entarity D eterm in in g R egions (CDRs! D is n lp h id e Bond
Heavy Chain