Final answer:
Haptens are indeed antigenic substances that can have reactivity but lack immunogenicity unless they are bound to a carrier molecule, which makes the statement true.
Step-by-step explanation:
The statement "Haptens are antigenic substances that have reactivity but lack immunogenicity" is true. A hapten is a small molecule that, by itself, cannot provoke a full immune response until it binds to a larger carrier molecule. Once attached, it forms a conjugate antigen, becoming immunogenic and capable of stimulating an immune response. Haptens are technically antigenic due to their ability to react with specific antibodies, but they are not immunogenic because they do not elicit an immune response without being bound to a larger carrier protein.
For example, the hapten urushiol is found in poison ivy and can cause an allergic reaction in the form of contact dermatitis when it binds to a protein in the skin. Similarly, the hapten penicillin can cause allergic reactions in individuals sensitive to drugs in the penicillin family.