The number of leukocytes per microliter of blood, normally ranging from 4,000-11,000 cells/μL. This count helps diagnose infections, immune disorders, and blood cancers.
Named for their colorless appearance under microscopy, contrasting with red blood cells. The term 'white' is somewhat misleading as these cells are actually transparent, but the name stuck from early microscopic observations in the 1840s.
White blood cells are actually outnumbered 700:1 by red blood cells, yet they're your body's entire defense force! When you get an infection, your bone marrow can increase white cell production by 10-fold in just hours - like mobilizing a massive army.
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