Or leukocyte; One of a number of different cells that play a part in the body's defenses and give immunity against disease. Some (phagocytes and macrophages) engulf invading microorganisms, others kill infected cells, while lymphocytes produce more specific immune responses. White blood cells are colorless, with clear or granulated cytoplasm, and are capable of independent ameboid movement. They occur in the blood, lymph and elsewhere in the body's tissues.
Unlike mammalian red blood cells, they possess a nucleus. Human blood contains about 11,000 leukocytes to the cubic millimeter —about one to every 500 red cells.
White blood cell numbers may be reduced (leucopenia) by starvation, pernicious anemia, and certain infections, such as typhoid and malaria. An increase in their 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.