Paratope

An antibody with a circled region depicting where the paratope is found.
1. Antigen-binding fragment (Fab)
2. Antibody crystallizable region (Fc)
3. Heavy chains
4. Light chains
5. Variable region of the antibody. The paratope is the key-shaped section that makes direct contact with the antigen.
6. Hinge regions

In immunology, a paratope, also known as an antigen-binding site, is the part of an antibody which recognizes and binds to an antigen. It is a small region at the tip of the antibody's antigen-binding fragment and contains portions of the antibody's heavy chain and light chain. Each paratope is a set of six complementarity-determining regions - three on the light chain and three on the heavy chain - that extend from a fold of anti-parallel beta sheets. Each arm of the Y-shaped antibody has an identical paratope at the end.

B-cell receptors include antibodies, so they include paratopes. All of the paratopes on any individual B cell are identical. The uniqueness of a paratope allows it to bind to only one epitope with high affinity and, as a result, each B cell can only respond to one epitope. The binding of paratopes on B-cell receptors to their specific epitope is a critical step in the adaptive immune response.