|MadSci Network: Medicine|
Actually, they're not white or purple--they're colorless! The red blood cells are red, of course, and they give blood its red color. But the white blood cells have very little color themselves.
When scientists look at blood under a microscope, they first treat it with a stain. The stain has dyes that turn the blood cells different colors. This makes it easier to see the different parts of the blood cells. Without the stain, it would be hard to see the white blood cells at all.
Usually, blood is treated with a Wright stain, which contains a dye called methylene blue. This makes parts of the white blood cells look purple!
Thanks for your question.
Stephen C. Lattanzi, M.D.
Dorland's Medical Dictionary. W.B. Saunders, 2002.
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