A 10-year-old had an immediate reaction with hives and breathing difficulty after receiving a medication. Four weeks later he develops a delayed skin reaction to a different exposure. What type of hypersensitivity is the SECOND reaction?

Correct answer: A. Type IV, cell-mediated (delayed) hypersensitivity

• A. CORRECT. A reaction developing DAYS to WEEKS after exposure, without urticaria or airway involvement, is a TYPE IV delayed hypersensitivity mediated by sensitized T LYMPHOCYTES and macrophages — no antibody involved. The classic dental examples are contact dermatitis from latex accelerators or nickel, contact stomatitis, and erythema multiforme. It peaks at 48–72 hours and is the mechanism of the tuberculin skin test.
• B. Type II involves IgG or IgM directed against cell-surface antigens, as in autoimmune hemolytic anemia or pemphigus.
• C. Type III involves circulating antigen-antibody complexes depositing in tissue, as in serum sickness or lupus nephritis.
• D. Type I is IgE and mast-cell mediated and occurs within MINUTES — this describes the first reaction, not the second.

Note: Latex allergy has BOTH forms: Type I to latex proteins (dangerous, immediate) and Type IV to the rubber accelerators (contact dermatitis, delayed). They are different allergies to the same glove.