Which statement best describes antibody structure and specificity?

• A. Antibodies are composed of nucleic acids and do not interact with antigens.
• B. Antibody "Y" shape. The epitopes (binding sites) recognize and bind only to certain antigens. Antibodies recognize only one kind of antigen.
• C. Antibodies are enzymes that catalyze antigen degradation.
• D. Antibodies function as lipids that store energy.

Answer: (B)

Antibodies derive their specificity from their structure. They are Y-shaped proteins made of two heavy chains and two light chains, linked by disulfide bonds. The tips of the Y—the variable regions from both chains—form the antigen-binding site, or paratope. This site is shaped and chemically matched to a specific part of an antigen called an epitope. Because of this precise fit, the antibody binds only to that particular antigen (or a very similar one), giving the immune response its high selectivity. The interaction relies on noncovalent forces and is reversible, allowing the antibody to recognize and respond to diverse invaders. This focus on a specific antigen contrasts with descriptions that treat antibodies as nucleic acids, enzymes, or lipids, which they are not.