What are the small parts of an antigen that trigger an immune response called?

• A. Antigens
• B. Epitopes
• C. Immunogens
• D. Reactants

Answer: (B)

The small parts of an antigen that trigger an immune response are referred to as epitopes. Epitopes, also known as antigenic determinants, are specific regions on the surface of an antigen that are recognized and bound by antibodies or by specific receptors on T cells. Each epitope is unique and can stimulate an immune response, prompting the production of antibodies or activation of T cells that target that particular antigen.

Antigens are larger molecules that can contain multiple epitopes. While they are responsible for the overall recognition by the immune system, it is the epitopes that specifically interact with immune receptors. Immunogens are a subset of antigens capable of inducing an immune response but do not specifically define the part of the antigen that elicits that response. Reactants is not a term typically used in immunology to describe any components related to immune responses. Hence, epitopes are the precise elements crucial for initiating and directing the immune response.