Which cell type begins to infiltrate the myocardium around 24 hours after a myocardial infarction?

• A. Neutrophils
• B. Macrophages
• C. Lymphocytes
• D. Eosinophils

Answer: (A)

The timing of the inflammatory response after a myocardial infarction is being tested. Neutrophils are the earliest white blood cells to respond to tissue injury. They begin to infiltrate the damaged myocardium around 24 hours after the infarction, peaking in the first couple of days. This rapid arrival helps clear dead cells and debris through phagocytosis and the release of enzymes, but it can also contribute to additional tissue injury if unchecked. Macrophages, by contrast, arrive a few days later (roughly 3–5 days) to clean up debris and promote repair, while lymphocytes come in later still. Eosinophils are not a primary feature of the early post-infarct inflammatory response. So, the cell type that begins to infiltrate around 24 hours is neutrophils.