Abstract
Eosinophils are circulating granulocytes with a bilobed nucleus and a cytoplasm that characteristically stains pink with eosin. These cells normally constitute 1%–2% of circulating leukocytes. However, a large proportion of the eosinophils of the body are resident in walls of the gastrointestinal tract. The bone marrow also contains a reserve of immature and mature eosinophils. Eosinophils are thought to have evolved as effector cells in host defence reactions to helminth parasites, and these cells contain cationic granular constituents that can induce injury to worms. Infections by such organisms are associated with a blood eosinophilia and the recruitment of large numbers of cells at the site of infection. Thus, the immune system can be programmed, characteristically involving lymphocyte polarisation towards Th2 cells, to recognise the presence of the parasite. Worm antigens can trigger degranulation of tissue mast cells by cross-linking cell-bound IgE and the recruitment of appropriate leukocytes, where eosinophils are predominant. It is thought that allergic diseases, e.g. allergic asthma, are an aberration of this natural defence reaction, where otherwise harmless agents in the environment trigger the same chain of events in susceptible individuals.
Published Version
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