Phagocytes

White blood cells play fundamental roles in the defence against invading microorganisms and the recognition and destruction of neoplastic cells. These cells also participate in acute inflammatory reactions. The phagocytes that are discussed in this chapter include neutrophils, eosinophils, monocytes/macrophages and basophils/mast cells. These phagocytic cells are each generated from pluripotent haematopoietic stem cells which are located within the bone marrow. Development of these various classes of white blood cells is a consequence of specific lineage specific cellular hierarchies which is governed by the intrinsic genetic programs provided by these primitive haematopoietic cells and by external stimuli including particular combinations of cytokines, matrix proteins and additional non-haematopoietic cells that reside within the marrow environment. Orderly function of these pathways leads to the generation of a normal constellation of these haematopoietic cells. These tightly controlled biological processes are required for normal life while alterations in various aspects of these regulatory pathways lead to disease in man. An understanding of the normal biology of each of these phagocytic cells provides insight into a number of these clinical disorders that are associated with congenital or acquired disorders of production or function of these various classes of phagocytes.