Which of the following substances causes an inappropriate immune response?

• A. Haptens
• B. Exogenous pyrogens
• C. Endogenous pyrogens
• D. Allergens

Answer: (D)

Allergens are substances that provoke an inappropriate immune response in susceptible individuals. In allergic people, these harmless environmental or dietary components trigger Type I hypersensitivity. The process starts with sensitization, where the immune system learns to produce allergen-specific IgE that binds to mast cells and basophils. Upon subsequent exposure, the allergen cross-links the bound IgE, causing cells to release histamine and other mediators that produce symptoms like itching, swelling, runny nose, bronchoconstriction, or hives. This misdirected reaction distinguishes allergens from normal defenses and makes them the classic cause of an inappropriate immune response.

Haptens, by themselves, are not inherently immunogenic; they must attach to a carrier protein to become capable of provoking an immune response. Pyrogens (exogenous or endogenous) trigger fever through cytokine signaling, reflecting an immune activation rather than the allergic misresponse seen with allergens.