Antigen is which of the following?

• A. Receptor on B cell
• B. Substance causing allergic reaction
• C. Protein produced by T cells
• D. Substance triggering immune response

Answer: (D)

An antigen is any molecule or part of a molecule that the immune system can recognize and bind to with antibodies or T cell receptors, triggering an adaptive immune response. Recognition is highly specific, so antibodies and T cells respond to particular structures on the antigen. Pathogen components like proteins or polysaccharides are common antigens, but antigens can also be other molecules; small molecules (haptens) become immunogenic when attached to a carrier. Allergens are special kinds of antigens that provoke allergic reactions, but not all antigens cause allergy. The key idea is that the substance activates the immune response.