The portion of an antigen that binds to an antibody or T-cell receptor is called what?

• A. Allergin
• B. Avidin
• C. Epitope
• D. Valence

Answer: (C)

An epitope, or antigenic determinant, is the specific region of an antigen that is recognized by immune receptors. The antibody binds to this region with its complementary paratope, and T-cell receptors recognize peptide epitopes presented by MHC molecules. Epitopes can be linear (a continuous sequence of amino acids) or conformational (formed by the protein’s 3D shape bringing distant residues together). An antigen can have multiple epitopes, which is why different antibodies or T-cell receptors can target the same molecule in different ways. The other terms don’t describe the binding region: a allergen (allergenic substance) is what can trigger an allergic response but isn’t the binding site itself; avidin is a biotin-binding protein with no relation to antigenic determinants; valence refers to the number of binding sites or epitopes available, not the specific part that binds.