the.com/white blood cell

means A colorless blood cell that defends the body by hunting, engulfing, or destroying germs, infected cells, and other invaders.

from Straightforward descriptive naming: these cells lack the red, oxygen-carrying hemoglobin of red blood cells, so under early microscopes they simply looked pale or colorless — 'white' by contrast, not by any pigment of their own. The scientific term 'leukocyte' tells the same story in Greek, from leukos (white) and kytos (cell/vessel), coined in the 1800s as microscopy and cell theory advanced and scientists began classifying blood's different cellular residents.