Michel Rabinovitch and/or produce cytokines that can modify their behaviour. The molecular analysis of endocytosis was aided dramatically by the use of mutated ligands and receptors in transfectedfibroblasts or epithelial cells’; similar approaches are now being applied to study the role in phagocytosis of Fc receptors, which bind to antibody-coated particles and trigger their phagocytosiQ.

Professional phagocytes Phagocytosis in mammals is mainly the work of PMNs, monocytes and macrophages. These cell types have often been referred to as ‘professional phagocytes’. PMNs and monocytes arise from bone marrow precursors in response to haematopoietic factors, remain in the circulation for less than a day and actively move across the blood vessels into tissues in response to chematactic factors. They rapidly accumulate at sites of tissue damage, infection or inflammation. PMNs are terminally differentiated and survive in tissues for only one or two days, but monocytes that reach the extravascular compartment generally differentiate into macrophager Depending on their location in the body and on the functional demands placed on them, macrophages can live for days, weeks, months or even years. In addition to their role in phagocytosis, professional phagocytes secrete a variety of important molecules, including cytotoxic radicals of oxygen and nitrogen, enzymes that degrade the extracellular matrix, lipid mediators of inflammation, and cytokines that can modify the behaviour of phagocytes and several other cell types. The production of many of these secretory products is often stimulated by lipopolysaccharides of Gram-negative organisms. There are numerous studies of the normal course of phagocytosis of relatively inert, degradable or non-degradable particles such as latex beads, opsonized or denatured erythrocytes, yeast derivatives, or killed or live bacteria by professional phagocytes. Although these are taken up by the general mechanism outlined at the start of the article, the rate of maturation of the phagosomes and/or the nature of the terminal phagocytic compartments may depend on the type of particle