The cellular and humoral immune response to Schistosoma mansoni infections in inbred rats : I. Mechanisms during initial exposure

Fischer rats were infected with Schistosoma mansoni by exposure to cercariae. The effects of animal age at exposure and the size of that exposure were determined by quantitation of subsequent worm burdens. Optimal conditions for assay of protection mediated by cells or serum transferred from exposed donors were ascertained. The major initial protective immune response to infection was mediated by thymusdependent lymphocytes as demonstrated by discrete subpopulation cell transfer experiments. Simultaneously harvested serum enhanced worm survival and obviated the protective effects of transferred cells when serum and cells were given concomitantly. Subsquently, host immunity became dependent on antibody formation. In the chronic phase of infection, a small number of surviving parasites coexisted with antibody which specifically interacted with schistosomal antigens but did not protect in transfer experiments. These studies suggest a form of immunologic enhancement with antibody modulation of cellular immunity as possible components of the immune response to both acute and chronic schistosomiasis.