Name the four types of hypersensitivity and a brief example of each.

• A. Type I (immediate, e.g., anaphylaxis); Type II (cytotoxic, e.g., ABO incompatibility); Type III (immune complex, e.g., serum sickness); Type IV (delayed-type, e.g., contact dermatitis)
• B. Type I (delayed); Type II (cytotoxic); Type III (immune complex); Type IV (delayed)
• C. Type I (immediate); Type II (antibody-mediated); Type III (immune complex); Type IV (cell-mediated)
• D. Type I (immediate); Type II (cytotoxic); Type III (immune complex); Type IV (immediate)

Answer: (A)

Hypersensitivity reactions are categorized by how the immune response is triggered and how quickly it appears. The first type is immediate and IgE-mediated, causing rapid release of mediators from mast cells and leading to reactions like anaphylaxis. The second type is cytotoxic, with antibodies such as IgG or IgM binding to cell-surface antigens and directing cell destruction, as seen in ABO blood group incompatibility during transfusion. The third type involves immune complexes formed by antibodies binding soluble antigens; these complexes deposit in tissues and activate complement, producing inflammation, as in serum sickness. The fourth type is delayed-type, driven by T cells rather than antibodies, and develops over one or more days, with contact dermatitis being a classic example. This framework—immediate IgE-mediated, cytotoxic antibody-mediated, immune complex–mediated, and delayed T cell–mediated—fits the standard definitions and the provided example set best. Other options mix up the timing or mediators (for example, labeling the first type as delayed or the fourth as immediate), which does not align with the established classifications.