There are four major blood groups, whose compatibility or incompatibility is an important consideration in successful blood transfusion . An extraordinary and prolonged proliferation of leukocytes is known as leukemia . This overproduction suppresses the production of normal blood cells. A dramatic fall in levels of certain white blood cells occurs in persons with AIDS . Leukocytes as well as erythrocytes are formed from stem cells in the bone marrow.
Neutrophils constitute the vast majority of granulocytes.
They travel about by ameboid movement and can surround and destroy bacteria and other foreign particles. They release chemicals such as histamine and play a role in the inflammatory response to infection. The agranulocytes include the monocytes and the lymphocytes. Monocytes are derived from the phagocytic cells that line many vascular and lymph channels, called the reticuloendothelial system. They move to areas of infection, where they are transformed into macrophages, large phagocytic cells that trap and destroy organisms left behind by the granulocytes and lymphocytes.