White |blood cells|, called ~leukocytes~, are outnumbered by the red |blood cells| 600 to 1. These cells are spherical in shape and slightly larger than red |blood cells|. There are five types of ~leukocytes~. Three of the five have a granular appearance. These are the neutrophils, eosinophils, and the basophils. The other two, the lymphocytes and monocytes, have smooth, non-granular bodies. The main function of the ~leukocytes~ is to provide a defense against "foreign" material (infectious agents, foreign bodies, abnormal |proteins|). In the presence of a foreign material, basophils and some lymphocytes erupt a spew out chemicals that cause inflammation, trapping the invader. The other ~leukocytes~ then take the foreign material into their own bodies and digest them. This process of digestion is called phagocytosis. The cells that digest microbes are called phagocytes. The most numerous of the phagocytes are the neutrophils. In addition to neutrophils, eosinophils, and monocytes, the body has other phagocytes that are not white |blood cells|. They are classed as reticuloendothelial cells, a type of connective tissue cells.
Lymphocytes are the smallest white |blood cells| and are a part of the immune mechanism. They form antibodies against disease. When microbes invade the body, lymphocytes begin to multiply and they become transformed ~plasma~ cells. Each microbe stimulates only one type of lymphocyte to multiply and form one type of ~plasma~ cell. The type of ~plasma~ cell formed is the type that can make a specific antibody to destroy the particular microbe that has invaded the body.
Red bone |marrow| continually produces white |blood cells|, except lymphocytes and monocytes, and keeps a reserve ready in case of need. Lymphocytes and monocytes are produced by lymphatic tissue located in the |lymph nodes| and |spleen|. When a parasite or virus invades and begins to colonize, the reserves of white |blood cells| are released and the manufacturing of large quantities of the appropriate white cells begins. It is this increased production that causes |fever|. Because white |blood cells| are very specific for various illnesses, their count can help doctors diagnose patients.
