The role of parasites in genetic susceptibility to allergy: IgE, helminthic infection and allergy, and the evolution of the human immune system.

There have been numerous studies in the mouse illustrating the dichotomy of T cell responses, with the common classification orchestrated around Th1 vs. Th2 responses. This classification is now widely applied to human disease as well and the generic conclusion is that the Th1 responses are more likely to occur secondary to specific microbiologic insult but also inflammatory responses. In contrast, the Th2 response is the prevalent response in subjects with atopy and allergic disease but is also the mechanism for protection against helminthic infections. Unfortunately, the paradigm of Th1 vs. Th2 is not as clear in the human as it is in mouse models. Even so, the immunological mechanisms responsible for IgE production that are protective in helminthic infections, i.e. Schistosoma, are similar to those for the production of specific IgE against allergens. In fact, there also appear to be associations in the memory T cell subpopulation CD4+CD45RO+ and the elicitation of IgE against both parasites and allergens. In this review, we present the overall contemporary scheme on the role of parasites in genetic susceptibility to allergic IgE, helminthic infections with specific discussion of its implications for the evolution of the human immune system.