A 25-year-old woman develops tender erythematous nodules on her shins bilaterally (erythema nodosum) 3 weeks after starting oral contraceptives. On biopsy, the septal panniculitis shows infiltration by lymphocytes and histiocytes but NO vasculitis and NO granulomas. Erythema nodosum represents which hypersensitivity mechanism?
- A Type I (immediate) hypersensitivity — IgE-mediated mast cell degranulation
- B Type III (immune complex) hypersensitivity — complement-activating immune complex deposition
- C Type IV (delayed-type) hypersensitivity — T cell and macrophage-mediated inflammation in subcutaneous tissue ✓
- D Type II cytotoxic hypersensitivity — complement-fixing IgG against subcutaneous fat antigens
Correct answer: C. Type IV (delayed-type) hypersensitivity — T cell and macrophage-mediated inflammation in subcutaneous tissue
Explanation
Erythema nodosum is a reactive panniculitis representing a delayed-type (Type IV) hypersensitivity reaction to various triggers (drugs, Streptococcus, TB, sarcoidosis, OCPs, IBD). Histology shows predominantly lymphocytic and histiocytic infiltration in the septa of subcutaneous fat (septal panniculitis) without vasculitis or granulomas — consistent with cell-mediated immune activation.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.