Describe the mechanism of hapten-protein conjugates in drug allergy.

• A. Some drugs act as haptens; they bind proteins to form complexes recognized by the immune system, potentially causing T cell–mediated or IgE-mediated drug allergies.
• B. All drugs inherently cause IgG-mediated autoimmune reactions.
• C. Haptens are large molecules that trigger immune responses on their own without protein binding.
• D. Hapten-protein conjugates are irrelevant to drug allergy.

Answer: (B)

Haptens are small molecules that by themselves are not able to trigger an immune response. When such a drug binds covalently to a host protein, it creates a hapten-protein conjugate (a hapten-carrier complex) that the immune system can recognize as foreign. This conjugate forms a new combination antigen that is processed by antigen-presenting cells. The resulting hapten-modified peptides are presented to T cells, which can drive two main pathways of drug allergy.

In some individuals, B cells are helped by T cells to produce antibodies against the hapten-protein complex, including IgE in the case of immediate, IgE-mediated hypersensitivity. IgE bound to mast cells and basophils can be cross-linked when the drug hapten re-encounters the body, triggering release of histamine and other mediators that produce symptoms like rash, urticaria, or anaphylaxis. In other cases, a T-cell-mediated, delayed-type hypersensitivity reaction occurs, where sensitized T cells recognize haptenated peptides and secrete cytokines that recruit macrophages and other inflammatory cells, leading to dermatitis or systemic drug reactions.

Some drugs act as prohaptens and become reactive only after metabolic activation, forming covalent adducts with proteins that similarly generate these neoantigens and potential allergy. The key idea is that drug haptens by themselves are not immunogenic; their ability to bind proteins creates new antigens that the immune system can target, underlying both IgE- and T cell–mediated drug allergies.