Which term describes the specific parts of an antigen that stimulate immune responses?

• A. Regions
• B. Antibodies
• C. Antigens
• D. Epitopes

Answer: (D)

Epitopes are the precise regions on an antigen that are recognized by antibodies or T-cell receptors and thus trigger an immune response. An antigen can have multiple epitopes, which is why a single molecule may be targeted by different antibodies. These contact sites can be linear, consisting of a sequence of amino acids, or conformational, formed by the protein’s three-dimensional folding. The other terms don’t fit: regions is too vague to specify where immune recognition occurs; antibodies are the immune proteins that bind epitopes but are not the sites themselves; antigens are the whole molecules that can provoke an immune response, not the specific stimulating parts.