Antigen binding site in an antibody is found between:

Antibodies are part of the humoral immune system. They are secreted by B lymphocytes. Antibodies circulate in the blood and recognize foreign antigens and neutralize them. The foreign antigens may be protein, carbohydrates, peptides, or nucleic acids of bacteria, microbes, viruses, and any other substance that activates the immune response.

Antigen and antibody interaction requires two specific sites on the antigen and the antibody. An epitope is a site on the antigen that recognizes by the antibody. The site on the antibody that is responsible for the antigen-antibody interaction is called the antigen-binding site or paratope.

The paratope is a small region found on the antigen-binding fragments (Fab) of an antibody. The paratope is consists of a part of the heavy chains and light chains of the antibody. The paratope is identical at both ends of the Y-shaped antibody.