A characteristic sequence of events has been identified by phase-contrast and electron microscopy during antibody-dependent, eosinophil-mediated damage to schistosomula of Schistosoma mansoni in vitro. Human eosinophils initially adhere to the intact schistosomulum and then, in the presence of antibody, flatten and spread very intimately over the parasite's surface. Subsequently, dense material similar to the contents of the lysosomal granules of the eosinophils appears in the extracellular space between the eosinophil and the schistosomulum, probably following fusion of the granules with the plasma membrane of the cell. Eventually all the eosinophils adhering to the parasite are completely degranulated and large amounts of the dense material are observed on the surface of the schistosomulum. This release of granular material from the eosinophils is followed by structural changes in the schistosomulum, starting with vacuolation of the inner layer of the tegument, followed by removal of the tegument, often in the form of large sheets. Subsequently the tegument disintegrates and the fragments are phagocytosed by other eosinophils which have not degranulated. Eosinophils then attach to the exposed muscle layers of the schistosomula and participate in the further degradation of the parasites by phagocytosing fragments of muscle fibres and other cellular components. This sequence of events is compared with published observations of the damage induced by various combinations of antibody, complement and effector cells in vitro, and of cell-mediated damage to schistosomula in vivo, and it is concluded that the observations described in the present paper may reflect a process of destruction of schistosomula in the immune host.

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