Once the outer epithelial barrier of the body has been breached, phagocytosis by phagocytic ceils is the most important part of the host defense against invading microorganisms (Stossel, 1974; Klebanoff and Clark, 1978). There are two different phagocytic cell types involved in the process of phagocytosis: polymorphonuclear leukocytes (PMNLs) and mononuclear leukocytes (MNs: monocytes and macrophages). Both cell types are derived from precursor cells in the bone marrow. PMNLs (primarily neutrophilic granulocytes) are released into the circulation in vast numbers in a fully differentiated state, with a half life in the bloodstream of only about six hours (Cartwright et al., 1964). The turnover of these cells is enormous; each day about 108 PMNLs disappear from the bloodstream and the same number enters the bloodstream from the bone marrow. MNs are capable of further differentiation (van Furth, 1970). As soon as they leave the circulation they become macrophages of the mononuclear phagocytic system. These fixed macrophages are strategically placed to defend the body against invading microorganisms, by lining the blood sinusoids in the liver, spleen, bone marrow, adrenals, etc. After invasion of the tissues by microorganisms, also the PMNLs leave the circulation and move towards the vicinity of the invading microorganisms. This directional movement of the phagocytic cells is called chemotaxis.