A white blood cell that defends the body against infections and foreign substances. Types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils, each with specific immune functions.
From Greek 'leukos' (white) + 'kytos' (cell), coined in 1855. The name means 'white cell,' though these cells are actually colorless and appear white only in contrast to red blood cells under microscopy.
Leukocytes are like a specialized military with different units - neutrophils are the rapid-response infantry, lymphocytes are intelligence and special forces, and monocytes are the heavy artillery that clean up battlefields! Despite being called 'white,' they're actually transparent and only look white next to red blood cells.
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