Which of the following substances are not antigenic by themselves but can become antigenic if they attach to body proteins?

• A. Allergens
• B. Histamines
• C. Haptens
• D. Perforins

Answer: (C)

Small molecules that are too tiny to provoke an immune response can become antigenic when they attach to a larger protein carried in the body. These are haptens. By binding to a carrier protein, the hapten forms a hapten-carrier complex that presents new epitopes the immune system can recognize, triggering the production of antibodies. This is a key idea in immunology and in immunohistochemistry, where haptens are often linked to carriers to generate or detect specific antibodies. The other options don’t fit this pattern: allergens are antigens that can provoke an immune response on their own; histamines are mediators, not antigens; perforins are cytotoxic proteins, not antigens.