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White blood cells, or leukocytes, are found in the blood and lymph nodes and act as part of the body's immune system. They are produced in the bone marrow.

The number of white cells in the blood, which can be determined by a blood count, is an indication on whether the body might be fighting an infection (higher than normal), or may be immunocompromised (lower than normal).

There are six different types of white blood cells. Two make up over 90% of the white blood cells in the body. The different types are used to fight bacterial and fungal infections (the death of these cells forms pus), parasites, the histamine reaction, and to create antibodies.

White blood cell at Wikipedia