Paratope vs. Epitope

Paratope

A paratope, also called an antigen-binding site, is a part of an antibody which recognizes and binds to an antigen. It is a small region (of 5 to 10 amino acids) of the antibody’s Fv region, part of the fragment antigen-binding (Fab region), and contains parts of the antibody’s heavy and light chains. Each arm of the Y shape of an antibody monomer is tipped with a paratope, which is a set of complementarity determining regions.

The part of the antigen to which the paratope binds is called an epitope. This can be mimicked by a mimotope. The figure given on the right hand side depicts the antibody commonly found on a B leukocyte. The engraved inner portions of idiotype (encircled region no.5 in the above diagram) is the paratope where the epitope of the antigen binds.

Epitope

An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. For example, the epitope is the specific piece of the antigen to which an antibody binds. The part of an antibody that binds to the epitope is called a paratope. Although epitopes are usually non-self proteins, sequences derived from the host that can be recognized (as in the case of autoimmune diseases) are also epitopes.

The epitopes of protein antigens are divided into two categories, conformational epitopes and linear epitopes, based on their structure and interaction with the paratope. A conformational epitope is composed of discontinuous sections of the antigen’s amino acid sequence. These epitopes interact with the paratope based on the 3-D surface features and shape or tertiary structure of the antigen. The proportion of epitopes that are conformational is unknown.

By contrast, linear epitopes interact with the paratope based on their primary structure. A linear epitope is formed by a continuous sequence of amino acids from the antigen.

Epitope (noun)

the part of an antigen molecule to which an antibody attaches itself.