A 22-year-old medical student receives penicillin and 72 hours later develops fever, skin rash, and lymphadenopathy. Complement levels (C3, C4) are low. This reaction is best classified as:
- A Type I (anaphylactic) hypersensitivity — IgE-mediated
- B Type II (cytotoxic) hypersensitivity — IgG/IgM against cell antigens
- C Type III (immune complex) hypersensitivity — serum sickness-like ✓
- D Type IV (delayed-type) hypersensitivity — T-cell mediated
Explanation
Serum sickness reaction is the prototype of type III (immune complex-mediated) hypersensitivity: antigen-antibody complexes are deposited in vessel walls, activating complement (consuming C3 and C4) and triggering neutrophil-mediated tissue injury. The delay of 7-14 days (here presenting at 72 hours as an early form) reflects time needed to generate antibodies; fever, rash, arthralgias, and lymphadenopathy are classic features. Type I is immediate IgE-mediated. Type II targets specific cell/tissue antigens. Type IV peaks at 48-72 hours but does not lower complement.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.