The eosinophils normally constitute about 2 percent of all the blood leukocytes. Eosinophils are weak phagocytes, and they exhibit chemotaxis, but in comparison with the neutrophils, it is doubtful that the eosinophils are significant in protecting against the usual types of infection.
Eosinophils, however, are often produced in large numbers in people with parasitic infections, and they migrate into tissues diseased by parasites. Although most parasites are too large to be phagocytized by eosinophils or any other phagocytic cells, eosinophils attach themselves to the parasites by way of special surface molecules and release substances that kill many of the parasites. For instance, one of the most widespread infections is schistosomiasis, a parasitic infection found in as many as one third of the population of some developing countries in Asia, Africa, and South America; the parasite can invade any part of the body. Eosinophils attach themselves to the juvenile forms of the parasite and kill many of them. They do so in several ways: (1) by releasing hydrolytic enzymes from their granules, which are modified lysosomes; (2) probably by also releasing highly reactive forms of oxygen that are especially lethal to parasites; and (3) by releasing from the granules a highly larvacidal polypeptide called major basic protein.
In a few areas of the world, another parasitic disease that causes eosinophilia is trichinosis. This disease results from invasion of the body’s muscles by the Trichinella parasite (“pork worm”) after a person eats undercooked infested pork.
Eosinophils also have a special propensity to collect in tissues in which allergic reactions occur, such as in the peribronchial tissues of the lungs in people with asthma and in the skin after allergic skin reactions. This action is caused at least partly by the fact that many mast cells and basophils participate in allergic reactions, as discussed in the next paragraph. The mast cells and basophils release an eosinophil chemotactic factor that causes eosinophils to migrate toward the inflamed allergic tissue. The eosinophils are believed to detoxify some of the inflammation inducing substances released by the mast cells and basophils and probably also phagocytize and destroy allergen-antibody complexes, thus preventing excess spread of the local inflammatory process.
