Leukocyte

A leukocyte is a white blood cell (leukos = white).

Leukocytes are produced by the bone marrow and are found in the blood, lymph, lymphoid organs and some connective tissues. Their name comes from centrifugation of blood samples, and the red layer contains the erythrocytes, the transparent layer is plasma and the intermediate white layer contains the leukocytes.

Leukocytes play a very important role in immune defences. There are 4,000 to 11,000 leukocytes in a microlitre (a thousandth of a millilitre) of blood.

Leukocytes contain three main families of cells, depending on their morphology and function:

• monocytes, which have a phagocytosis function ;
• lymphocytes, which are involved in specific antigen recognition;
• and granulocytes, which are involved in bacterial and parasitic infections and in allergies.