Final answer:
An epitope is the area on an antigen that an antibody contacts. It's a small region, often containing only four to six amino acids. Antibodies can be cross-reactive, binding to similar epitopes on different antigens.
Step-by-step explanation:
An epitope is specifically the area on an antigen which contacts an antibody. Moreover, antibodies can exhibit cross reactivity, which happens when they react to similar epitopes present on different antigens. Epitopes are usually composed of a small region of an antigen, consisting of about four to six amino acids that form a three-dimensional structure. The binding between an antibody and an epitope involves noncovalent interactions and is specific to the complementary structure of the epitope.
An antibody molecule typically has two arms, known as the Fab regions, with each arm containing a variable region responsible for binding to the epitope. This binding is necessary for the immune response, including pathogen neutralization and antibody-dependent cell-mediated cytotoxicity. Additionally, the term avidity refers to the overall strength of all interactions between an antibody and all the epitopes it binds to.