Which of these white blood cells migrate through tissue as macrophages and function to remove and destroy bacteria, damaged cells, and neoplastic cells?

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

Answer: (A)

Monocytes circulate in the blood as precursors and, when they move into tissues, differentiate into macrophages. These macrophages are large, highly phagocytic cells that ingest bacteria, dead or damaged cells, and neoplastic (cancerous) cells, helping to clean up debris and present antigens to activate the adaptive immune response. Eosinophils mainly handle parasitic infections and allergic reactions, not tissue macrophage function. Lymphocytes (B, T, NK cells) drive adaptive immunity rather than performing phagocytosis as macrophages. Neutrophils are rapid responders to acute bacterial infection and are highly phagocytic, but they do not mature into tissue macrophages and are short-lived. Therefore, the ability to migrate into tissue and become macrophages that remove bacteria, damaged cells, and neoplastic cells points to monocytes.