In acute myocardial infarction, coagulation necrosis becomes histologically apparent at 12–24 hours. At approximately what time post-infarction do neutrophil infiltration peak and then transition to macrophage-dominated phagocytosis?
- A Neutrophils peak at 6–12 hours; macrophages appear at 24–48 hours
- B Neutrophils peak at 1–2 hours; macrophages appear at 12–24 hours
- C Neutrophils peak at 24–72 hours; macrophages appear at 3–7 days ✓
- D Neutrophils peak at 1 week; macrophages appear at 2 weeks
Correct answer: C. Neutrophils peak at 24–72 hours; macrophages appear at 3–7 days
Explanation
After MI, the inflammatory sequence is well characterized: neutrophil infiltration begins at 12–24 hours and peaks at 24–72 hours, followed by replacement by macrophages (which phagocytose necrotic cardiomyocytes) beginning at 3–7 days. Granulation tissue forms from 10 days to 3 weeks. Dense collagenous scar (fibrosis) repletes the infarcted area by approximately 6–8 weeks. This timeline is important for determining approximate MI age at autopsy.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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