Final answer:
A hapten is a (4) small molecule that can bind to antibodies but is not immunogenic unless attached to a larger carrier protein, forming a conjugate antigen that can stimulate an immune response. Haptens are involved in certain allergic reactions, such as those caused by poison ivy or penicillin.
Step-by-step explanation:
A hapten is defined as a simple substance that is not immunogenic by itself but can react with a specific antibody. While haptens are too small to elicit an immune response alone, they can become antigenic when attached to a larger carrier molecule, typically a protein. Once bound to a carrier, the combined structure forms a conjugate antigen that can stimulate the production of antibodies. These antibodies can later interact with the free hapten molecules.
For example, molecules like urushiol, found in poison ivy, or penicillin can function as haptens leading to allergic reactions when they bind to proteins in the body. This process forms the basis for contact dermatitis in the case of urushiol, or drug allergies for penicillin. It's important to understand the distinction between a hapten and a full antigen, as haptens require a carrier molecule to become antigenic and provoke an immune response.