Which part of the antibody primarily determines its unique specificity?

• A. Heavy chains
• B. Light chains
• C. Short arms of the Y
• D. Constant region

Answer: (C)

Antibody specificity is defined by the variable regions that form the antigen-binding site at the tips of the Y. The binding site is created by the variable domains of both the heavy and light chains, with the hypervariable loops (CDRs) giving each antibody its unique shape and chemistry to recognize a specific antigen. The constant region at the stem is not involved in recognizing or binding the antigen; it governs effector functions like receptor binding and complement activation. So, while the Fab arms contain the binding site, it’s the variable sequences within those regions that determine what antigen is bound and how tightly.