Blood cell

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A blood cell (also called blood corpuscle) is any cell of any type normally found in blood. In mammals, these fall into three general categories:

Together, these three kinds of blood cells sum up for a total 45% of blood tissue (55% is plasma).

Comparison
cell type	Primary function	Lifetime
Red blood cells	transport of oxygen	120 days
White blood cells	produces antibodies to fight infection	days to years
Platelets	blood clotting	8 days

Disorders

A decrease in number of blood cells is called cytopenia. An increase, on the other hand, is called polycythemia.

What is Blood? from the Genetic Science Learning Center at the University of Utah.
Cells of the blood