Ultrastructural studies of eosinophils and their interaction with parasites

Data accumulated from both in vitro and in vivo studies indicate that eosinophils are preferentially attracted to helminth parasites. Indeed even when in the minority in a mixed leucocyte population they effect adherence considerably in advance of other cell types. Eosinophils are generally considered to be only weakly phagocytic compared to neutrophils, but it is now clear that they have developed a unique and specialized mechanism of degranulation for coping with targets too large to be phagocytosed. Eosinophil-mediated cytotoxicity in vitro has been shown to involve an oxygen-dependent microbicidal system together with the release of highly cationic proteins from the lysosomal secretion granules. In addition, cytotoxin-induced damage may be augmented by physical trauma if the eosinophils achieve invasion of the surface tissues of the parasite. Trematodes and cestodes appear to be more susceptible to eosinophil-mediated damage and killing than do nematodes, but if the granulocytes do accomplish perturbation of the resistant nematode cuticle, mortality ensues, often in the absence of cellular invasion. In vivo studies indicate that, although it is the eosinophil which initiates surface damage, a number of different cell types may interact sequentially to achieve elimination of the metazoan invader. Eosinophils have also been shown to phagocytose certain protozoan parasites in vitro, but the relevance of this phenomenon to the in vivo situation is at present unclear.