The role of macrophage activation in chronic inflammation.

The macrophage is the characteristic cell type in chronic inflammatory reactions, in the rheumatoid synovium, as in other sites. When macrophages are activated, considerable synthesis of enzymes and other proteins occurs. Macrophages can be activated by (i) products of activated lymphocytes, (ii) immune complexes and (iii) the complement cleavage product C3b. Among the many consequences of macrophage activation are (i) secretion of hydrolytic enzymes, (ii) cleavage of C3 into C3a, which is cytolytic, and C3b, (iii) production of tissue thromboplastin, a powerful procoagulant, and (iv) formation of polyamine oxidase, which in the presence of appropriate substrates generates factors that lyse or limit the proliferation of tumour cells, lymphocytes and micro-organisms. The relevance of these observations to the pathogenesis of chronic inflammatory reactions is discussed.