BASIC STRUCTURE OF IMMUNOGLOBULINS

All Igs have the same basic structural units of 2 identical light chains and 2 identical heavy chains, the heavy and light chains are joined together by inter chain disulphide bonds and non-covalent interactions. The number of inter chain disulphide bonds varies among different Igs. Within the polypeptide chains i.e. the heavy and light chains there are also present intra-chain disulphide bonds. Amino acid sequence of both heavy and light chains of an Ig characterizes two distinct regions of the chains based on variability of the amino acid sequence, known as VARIABLE (V) and CONSTANT (C) regions .Light and heavy chains are composed of both a variable and constant region designated VL and CL (light chains) and VH and CH (heavy chains).The amino acid sequence of the variable region form the N-terminal ends of the chains and determine antigenic specificity of the Igs. Constant regions are the same for each specific class of Ig and carry the effector sites.

 

Light chain- VL-about 100-110amino acids, CL-100-110 amino acids.

 

There are two types of light chains,
kappa and lambda,( κ and λ) the κ are twice as much as λ. There are also four classes of the λ chains.These chains weigh about 23KDa. Differences in the type of light chains also form a basis for grouping of Igs into various types. The variable region makes up half of the entire light chain and the constant region the remaining half.

Heavy chains- VH-110 amino acids, CH-330-440 amino acids. There are 5 types of heavy chains which defines the class of Igs, namely, Alpha, Gamma, Miu, Delta and Epsilon (α, γ,μ,δ,ε).the heavy chains are between 53-75KDa.the variable region makes up a quarter of the entire heavy chain while ¾ of the remaining chain is the constant region. The hinge region is the area of the Ig where the arms of the Abs form a ‘Y’,it is a flexible region. Igs also have domains formed from folds of the globular region containing the intrachain disulphide bonds and they are VL and CL (light chain domains) and VH and CH (heavy chain domains), seen in the three dimensional images of the Ig. The constant region of light chain and the appropriate heavy chain form globular constant domains while the variable regions of light chain 1 and corresponding heavy chain interact to form globular variable domain. Ig s also have attached to their CH oligosaccharides and in other cases these carbohydrates are attached to other areas. The variable regions of an Ig are also further divided into hypervariable or complementarity determining gions (CDRs) which distinguishes Abs with different specificities and is found on both light and heavy chains and the frame work regions lie between the CDRs. There are about 3 hyper variable regions on the VL and 4 on the VH, and these contribute to uniqueness of each antibody.