Parasitic protozoa and helminths are a diverse group of organisms which together form a major cause of infectious disease in humans and livestock. Studies in animal models have revealed that T lymphocytes and the cyto kines they produce play a crucial role in determining the outcome of parasitic infection in terms of both protective immunity and immuno pathology. Of particular interest is recent evidence that different parasitic infections in the context of different host genetic background can trigger polarized CD4+ T cell subset responses. The set of cytokines produced by these different T helper responses, in turn, can have opposing effects on the parasite, resulting in either control of infection or promotion of disease. Moreover, cytokines produced by one CD4+ subset can block either the