The Killing of Pathogens by Phagocytes

Phagocytes include neutrophils, eosinophils, and monocyte-macrophages. They are cells specialized for destruction. Their name reflects the fact that they routinely ingest large particles during the course of their activities (Gr. 0a:yw, I eat). The objects that they destroy include invading pathogens, damaged and foreign cells, and connective tissue. Small targets such as bacteria and protozoa are destroyed after phagocytosis, a process by which the target is internalized into a cytoplasmic vesicle (the phagocytic vesicle) lined with what was originally plasma membrane (I). Larger targets can be killed without internalization (2-4). Because many of the destructive agents em­ ployed by phagocytes are nonspecifically reactive, normal tissue will be destroyed, especially near a region of inflammation where phagocytes accu­ mulate in large numbers (5-7); this accounts for much of the pathology at sites of inflammation. Pathogens are killed by all three types of phagocytes, though different phagocytes seem to specialize in different types of patho­ gens. Neutrophils appear to have few other major functions, but eosinophils are thought to limit the extent of an allergic response by the destruction of mediators (8), while the mononuclear phagocytes are the major elements