Question

Which one of the following combination acts as a usual antigen binding site of an antibody?

• A. Variable regions of a light and another heavy chain
• B. Variable regions of two light chains
• C. Variable regions of two heavy chains
• D. Variable region of a heavy chain and constant region of a light chain

Answer 1

The correct option is A Variable regions of a light and another heavy chain

The antibody is a 'Y' shaped protein (also called immunoglobin) consisting of two identical heavy and two identical light polypeptide chains. These chains are held together by disulfide bonds.

This antibody is mainly divided into two regions:-

1. Fab Region: It is the antigen binding fragment i.e. it recognizes and binds to the specific antigen. This region consists of the variable and constant domain from each of the two heavy and light chain (figure).
   
2. Fc Region: It is the crystallizable fragment which consists of the constant domain from both heavy chains. This region is involved in identifying and binding to the Fc receptors on the cell surface (Eg. phagocytes) or to complement proteins to induce an appropriate immune response.

So, the correct answer is option A.