Which term is the trigger of an immune response?

• A. Antigen
• B. Antibody
• C. Allergen
• D. Sepsis

Answer: (A)

Antigen is the trigger because it is any substance that the immune system recognizes as foreign and that stimulates an immune response. The immune system detects specific features on antigens, called epitopes, using receptors on B cells and T cells (and, for some responses, antigen-presenting cells). This recognition leads to the production of antibodies or activation of T cells, forming the basis of humoral and cell-mediated immunity. Allergens are a subset of antigens that provoke allergic (IgE-mediated) reactions, so they can trigger responses, but the general initiator is the antigen. Antibodies are the products of the response, not the trigger, and sepsis describes a severe infection state that can provoke an immune reaction but isn’t the trigger term itself.