Which of the following statements is true aboutisotypic variation ?
Changes in heavy and light chain in constant region is responsible for class and subclass of immunoglobulins Antibodies are mucoproteins that are found in the gamma (y) globulin fraction of serum on electrophoresis. Structure of antibody The basic structural unit for each class is a four chain protein with two heavy (H) and two light (L) chains. These are polypeptide chains linked together by disulfide bonds. - L chain is attached to H chain by disulfide bond - Two H chains are joind together by disulfide bond Both (H) and (L) chains are made up of two regions i.e. variable and constant region. i) H chain - One variable and 3 constant region ii) L chain - One variable and one constant region Heavy chain There are five isotypes or classes of antibodies. The isotope variation or class of antibody is due to variation in the amino acid sequence in the constant region of the heavy chain. Because there are five potential variation in the arninoacid sequence in the constant region there are five classes of antibody i.e. IgG, IgM, IgA, IgD, IgE. These H chain differences are called isotypes and are designated by the greek letters gamma (y), alpha (a) mu (1.1), epsilon (e) and delta (8) respectively. Light chain All five classes or isotypes have the same amino acid sequence on the L chains. Two L chain isotypes designated kappa (lc) and lambda (X) exist for all five classes. Constant region The amino acid sequence in the constant regions of both H and L chains is similar for all antibody molecules within each class. Variable region The amino acid sequence of the variable regions on both H and L chains varies with the antigen towards which the paicular antibody is directed. Poion of the variable region which has highly variable amino acid sequence is called hypervariable region. The hypervariable regions of the H and L chains associate to form the epitope binding region known as antibody idiotype. (The pa of the antigen which binds to the antibody is called epitope). Sites on hypervariable region which make actual contact with the epitopes are called complementary determining regions (CDRS) or paratopes. The infinite range of antibody specificity of inzmunoglobulins depends on the variability of amino acid sequences at the variable unit of H and L chains. Isotype switching Isotype switching is a biological mechanism that changes a B cell's production of antibody from one class to another, for example IgM in primary response to IgG in secondary response. During this process the constant region poion of the antibody heavy chain is changed, but the variable region of the heavy chain stays the same. "Since the variable region does not change, class switching does not effect antigen specificity". Instead, the antibody retains affinity for the same antigen, but can interact with different effector molecules. Note No option amongst the given four is completely correct. Isotype variation is due to variations in amino-acid sequences in the constant region of carboxyterminal of heavy chains (not in both heavy and light chains).
These result due to subtle amino acid changes resulting from allelic differences
These result due to changes in amino acid in heavy and light chain at variable region
Changes in heavy and light chain in constant region is responsible for class and subclass of immunoglobulins
These are areas in antigen that bind specifically to antibody