In the past few years, considerable evidence has accumulated to suggest the existence of functionally polarized responses by the CD4+ T helper (Th)--and the CD8+ T cytotoxic (Tc)-cell subsets that depend on the cytokines they produce. The Th1 and Th2 cellular immune response provide a useful model for explaining not only the different types of protection, but also the pathogenic mechanisms of several immunopathological disorders. The factors responsible for the polarization of specific immune response into a predominant Th1 or Th2 profile have been extensively investigated in mice and humans. Evidence has accumulated from animal models to suggest that Th1-type lymphokines are involved in the genesis of organ-specific autoimmune diseases, such as experimental autoimmune uveitis, experimental allergic encephalomyelitis, or insulin-dependent diabetes mellitus. Accordingly, data so far available in human diseases favor a prevalent Th1 lymphokine profile in target organs of patients with organ-specific autoimmunity. By contrast, Th2-cell predominance was found in the skin of patients with chronic graft-versus host disease, progressive systemic sclerosis, systemic lupus erythematosus, and allergic diseases. The Th1/Th2 concept suggests that modulation of relative contribution of Th1- or Th2-type cytokines regulate the balance between protection and immunopathology, as well as the development and/or the severity of some immunologic disorders. In this review, we have discussed the paradigm of Th1 and Th2 cytokines in relation to autoimmunity and allergy.