The role of granulocytes in inflammation

Neutrophils secrete of variety of biologically active compounds, especially when they accumulate at sites of inflammation. Secretory products are delivered to the tissues both by exocytosis of cytoplasmic granules and by metabolic events taking place at the plasma membrane. The release of lysosomal constituents, such as lactoferrin, elastase and collagenases, is associated with the regulation of the turnover of neutrophils, their participation and activity in the inflammatory reaction, and breakdown of cartilage and connective tissues, for example. Generation of cytotoxic oxygen radicals and compounds, e.g. the superoxide anion, hydrogen peroxide and the hydroxyl radical, is initiated by many inflammatory mediators. These two systems, either individually or in collaboration, can cause damage to many types of structures. For instance, when endothelial cells are injured, increased vascular permeability may occur. If such injury involves the pulmonary capillary system a respiratory distress syndrome may supervene. Leukotrienes are potent mediators of inflammation, formed in neutrophils after exposure to various other chemotactic or perturbating compounds. Leukotriene B4 is the most potent of the hitherto described compounds, being a promotor of neutrophil adherence, aggregation and chemotaxis in vitro of similar potency as the formylated synthetic chemotactic peptides, e.g. fMLP, and as the C5a fragment. However, the ability of LTB4 to induce a release of lysosomal enzymes is only half of that of fMLP, and, finally, the capacity to initiate a chemiluminescence response, being a measure of the oxidative metabolism, is only one-tenth of that of fMLP. Thus, leukotrienes of the B series seem to be a signal system whereby activated neutrophils can recruit cellular reinforcements, and, possibly, to act as an intracellular, second messenger system.