Which test counts white blood cells to assess infection or immune status?

• A. White blood cell count
• B. Hematocrit
• C. Platelet count
• D. Red blood cell count

Answer: (A)

Counting white blood cells directly reflects the quantity of leukocytes, the immune system’s main defenders. When infection or inflammation is ongoing, the body often increases white blood cells to fight it, a rise called leukocytosis. Conversely, infections that suppress the marrow or certain conditions can lower white blood cell numbers, known as leukopenia. Because this test specifically measures the immune cell population, it serves as a direct indicator of infection activity or immune status and is typically included in a complete blood count. The other options assess different blood components—hematocrit estimates the proportion of red blood cells, platelet count looks at clotting cells, and red blood cell count tallies the red cells themselves—so they don’t provide a direct readout of immune function.