Paratope

Paratopes make up the parts of the B-cell receptor that bind to and make contact with the epitope of an antigen. All the B-cell receptors on any one individual B cell have identical paratopes. 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 paratopes on B-cell receptors binding to their specific epitope is a critical step in the adaptive immune response.

Design of paratopes between species

The design and structure of paratopes can differ greatly between different species. In jawed-vertebrates, V(D)J recombination can result in billions of different paratopes. The number of paratopes, however, is limited by the composition of the V, D, and J genes and the structure of the antibody. Thus, many different species have developed ways to bypass this restriction and increase the diversity of possible paratopes.

In cows, an extra-long complementarity-determining region is considered to have an essential role in diversifying paratopes. Additionally, both chickens and rabbits use gene conversion to increase the number of paratopes that are possible.