Observations have been made on the protective mechanisms which appear to be involved in conferring a high degree of acquired resistance on Macaca mulatta within 2 years after initial exposure to Schistosoma mansoni. It has been shown that the resistant monkey develops marked cellular responses in the skin and lung during transit of schistosomula, and that many of these larvae are trapped and destroyed in the lung, where they elicit inflammatory foci of distinctive morphology which are described here in detail and named “tuft-like foci”. Both the skin and the lung reactions of the resistant monkey are indicative of hypersensitivity, but only the latter appears to retain schistosomula in considerable number. In spite of lung trapping, a considerable proportion of infective larvae survives the lung stage and reaches the hepatic portal blood, but these parasites are stunted and die within 60 days after challenge without further cellular reaction except that which occurs after their death in the liver. Circumlarval precipitates were not seen around any of the multiple trapped larvae examined. The findings are discussed in the light of the literature, and it is suggested, as a working hypothesis, that (a) both humoral and cellular factors are involved in acquired resistance and (b) schistosomes may be capable of partially blocking host antibodies, possibly through absorptive control.
Present address: Department of Pathology, Peter Bent Brigham Hospital, and Harvard Medical School, Boston, Massachusetts.