leukocyte

White blood cell. Leukocytes play a part in the body's defences and give immunity against disease. There are several different types. Some (phagocytes and macrophages) engulf invading micro-organisms, others kill infected cells, while lymphocytes produce more specific immune responses. Leukocytes are colorless, with clear or granulated cytoplasm, and are capable of independent amoeboid movement. They occur in the blood, lymph, and elsewhere in the body's tissues.

Unlike mammalian red blood cells, leukocytes possess a nucleus. Human blood contains about 11,000 leukocytes to the cubic millimeter – about 1 to every 500 red cells.

Leukocyte numbers may be reduced (leukopenia) by starvation, pernicious anemia, and certain infections, such as typhoid and malaria. An increase in the numbers (leukocytosis) is a reaction to normal events such as digestion, exertion, and pregnancy, and to abnormal ones such as loss of blood, cancer, and most infections.