Which cell is a type of white blood cell that differentiates into macrophages when it leaves the bloodstream and enters tissues?

• A. Monocyte
• B. Neutrophil
• C. Eosinophil
• D. Lymphocyte

Answer: (A)

Blood monocytes travel in the bloodstream and, when they migrate into tissues during an infection or inflammation, they differentiate into macrophages. Macrophages are large, long-lived phagocytes that engulf pathogens and debris and can also present antigens to activate other immune cells. The other white blood cell types mentioned — neutrophils (quick responders that don’t turn into macrophages), eosinophils (parasite fighters and mediators of allergic responses), and lymphocytes (T, B, and NK cells of the adaptive immune system) — do not differentiate into macrophages.