When blood is separated, whether by means of a centrifuge or in the process of coagulation, there are three distinct components visible: the red corpuscles or erythrocytes, the pale yellowish plasma, and a greyish fraction, smaller in volume than the other two. This last one is called the buffy coat, which contains the white blood cells or leukocytes as well as the platelets involved with clotting. Ordinarily it comprises only a percent of the total by volume, but in cases of infection or of leukemia this fraction may increase. Besides diagnostic uses, it can be separated from the other fractions in donated blood by an apheresis device.

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