Evidence is accumulating to suggest the existence of polarized human T-cell responses, reminiscent of TH1 and TH2 subsets described for mouse T cells. Human TH1 cells preferentially develop during infections by intracellular bacteria and trigger phagocyte-mediated host defense, whereas TH2 cells, which predominate during helminthic infestations and in response to common environmental allergens, are responsible for phagocyte-independent host response. Human TH1 and TH2 cells exhibit not only different functional properties but probably also distinct surface markers; TH2, but not TH1, clones express membrane CD30 and release the soluble form of CD30, a member of the TNF receptor superfamily. The cytokine profile of "natural immunity" evoked by different offending agents in the context of different host genetic backgrounds appears to be the most critical factor in determining the phenotype of the subsequent specific response. IL-12 and IFN-alpha and gamma produced by macrophages and NK cells favor the development of TH1 cells, whereas the early production of IL-4 by a still-unidentified cell type favors the development of TH2 cells. Clearly, polarized human TH1 and TH2 responses not only play different roles in protection, they can also promote different immunopathological reactions. Strong and persistent TH1 responses seen to be involved in organ-specific autoimmunity, contact dermatitis, and some chronic inflammatory disorders of unknown etiology. In contrast, polarized TH2 responses favor a reduced protection against the majority of infectious agents (including HIV) and, in genetically predisposed hosts, are responsible for triggering of allergic atopic disorders.