What type of white blood cell (leukocyte) is typically the most abundant in normal circulating human blood, acting as the first-responder phagocyte against bacterial infections?
Answer and explanation
Correct answer: C. Neutrophils
- Step 1: Leukocytes are classified into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).
- Step 2: Neutrophils constitute approximately 50% to 70% of total circulating white blood cells in healthy adults; their multi-lobed nuclei and cytotoxic granules allow rapid migration to infection sites to phagocytose invading bacteria.
Answer
Neutrophils
Why not the others: 'Lymphocytes' Lymphocytes are the second-most abundant white blood cells (20% to 40%), responsible for adaptive viral and humoral immunity; 'Monocytes' Monocytes comprise roughly 2% to 8% of leukocytes and differentiate into macrophages in tissues; 'Eosinophils' Eosinophils comprise roughly 1% to 4% of leukocytes and specialize in combating parasitic worms and allergic reactions.
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