Question: what portion of the antibody binds to an antigen?
Answer: The paratope is the part of an antibody which recognizes an antigen, the antigen-binding site of an antibody. It is a small region (15–22 amino acids) of the antibody's Fv region and contains parts of the antibody's heavy and light chains. The part of the antigen to which the paratope binds is called an epitope.