Drug Toxicity: Allergic Reactions

Type I hypersensitivity is IgE-mediated and occurs when allergen exposure triggers crosslinking of IgE antibodies bound to mast cells and basophils. This crosslinking causes mast cell degranulation and histamine release, leading to vasodilation, edema, and inflammation. In severe cases, this cascade results in anaphylaxis, characterized by bronchospasm, hypotension, and cardiovascular collapse.

Type II hypersensitivity involves IgG and IgM antibodies targeting drug-bound cell surface antigens. This interaction activates the complement cascade, leading to cell lysis and destruction. The resulting conditions include hemolytic anemia from red blood cell destruction, thrombocytopenic purpura affecting platelets, and granulocytopenia reducing white blood cells.

Type III hypersensitivity occurs when IgG forms antigen-antibody complexes that deposit in tissues, particularly blood vessels, joints, and kidneys. These immune complexes activate complement proteins and recruit inflammatory mediators, causing widespread inflammation. Clinically, this manifests as serum sickness with fever, rash, arthralgia, and lymphadenopathy, typically appearing 1-3 weeks after drug exposure.

Type IV hypersensitivity is T-cell-mediated and involves small drug molecules called haptens binding to carrier proteins. Upon re-exposure, sensitized T cells release cytokines, recruiting immune cells and causing tissue damage. These delayed hypersensitivity reactions typically present 48-72 hours post-exposure as contact dermatitis, maculopapular eruptions, or severe conditions like Stevens-Johnson syndrome.

Histamine is released during mast cell and basophil degranulation triggered by IgE crosslinking. This histamine release causes vasodilation, increased vascular permeability, and edema. These effects produce clinical symptoms ranging from urticaria and angioedema to severe anaphylaxis with bronchospasm and cardiovascular collapse in extreme cases.

Haptens are small drug molecules that bind to carrier proteins, forming immunogenic complexes capable of stimulating T-cell activation. During initial exposure, these complexes sensitize T cells. Upon re-exposure to the drug, sensitized T cells recognize the hapten-protein complex and release cytokines, triggering immune cell recruitment and inflammatory tissue damage.

Type I reactions occur immediately upon allergen exposure, causing rapid anaphylaxis. Type III immune complex reactions develop over 1-3 weeks as complexes accumulate in tissues. Type IV delayed hypersensitivity reactions manifest 48-72 hours post-exposure. This variation in onset timing helps clinicians differentiate between hypersensitivity mechanisms and guides appropriate diagnostic and therapeutic interventions.