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:

• Red blood cell, whose main purpose is the transport of oxygen;
• White blood cell, which produces antibodies to fight infection;

• Platelet, which is actually a fragment of the large bone marrow cells known as megakaryocytes and are important in blood clotting.

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