Question

Antigen binding site in an antibody is found between
(a) Two light chains
(b) Two heavy chains
(c) One heavy and one light chain
(d) Both (b) and (c) depending on the nature of the antigen

Answer 1

Hint: Antibodies have antigen-binding sites to recognize a specific part of the antigen and they are required for presenting the antigen to other lymphocytes to process and/or eliminate them. For this, antibodies require specific regions in their ‘Y’-shaped chain to identify the antigen.

Complete step by step answer:
- The antigen-binding site or the paratope is the site where the antigenic determinant or the epitope binds to the antibody. It is composed of one constant and one variable domain of each of the heavy and the light chains.
- The site is a part of Fragment antigen-binding (Fab).
- Each arm of the Y structure is able to bind to an epitope on the antigen, between a heavy chain and a light chain.

- Antigen-binding sites display high amino acid sequence diversity within the variable regions of the antibody as well as the T-cell receptor.
So, the correct answer is ‘one heavy and one light chain’.

Additional Information:
- In any given antibody molecule, the heavy chains are identical to each other and so are the light chains. Each chain is encoded by different genes.
- The hinge region of the antibody facilitates the binding between the antigen and antibody by opening and closing.
- There are five types of antibodies - IgG, IgA, IgM, IgE, and IgD.

Note:
- Antibodies belong to the ‘globins’ class of proteins. They are also known as immunoglobulins.
- All antibodies have the same basic structure of four polypeptide chains.
- The tail of the antibody does not bind to antigens.
- The most abundant antibody is IgG (immunoglobulin G).