Th1/Th2 balance in infection.

Cytokines produced by T helper (Th) cells are of critical importance for the outcome of many infectious diseases. Producing the "right" set of cytokines in response to an infectious agent can be a matter of life or death. The Th1/Th2 dichotomy, although an oversimplification has proven useful in the analysis of immune responses to infections. In some infectious diseases, most notably leishmaniasis or infections with gastrointestinal helminths, one Th subset is indispensable for clearing the infection, whereas the opposite Th subset is detrimental. More frequently, both Th1 and Th2 responses are required at different time points to effectively eradicate an infectious agent. The granuloma responses to either Mycobacterium tuberculosis or Schistosoma mansoni provide illustrative examples and are discussed in this review. There is accumulating evidence for frequent coexpression of Th1 and Th2 cytokines during the in vivo immune response to infections. The mechanisms by which infectious agents modulate Th1/Th2 phenotype development are summarized here. Finally, we review here the current evidence for cytokine imbalances induced by infections as pathogenic or protective factors in autoimmunity and allergy.