Leishmania specific Th1/Th2 cells have been identified in humans as well as in mice. There is a correlation between the clinical outcome of the infection and the cytokine response profile. Generally, the production of Th2 cytokines leads to severe infection, whereas the production of Th1 cytokines leads to subclinical or mild infections. In mice, an infection leads to a polarisation of either Th1 or Th2 Leishmania antigen specific cells. In contrast, both Th1 and Th2 Leishmania antigen specific cells can be identified in humans cured from L. donovani infections. Theoretically, Th1 cells and Th2 cells mutually down-regulate each other. However, the presence of antigen specific regulatory T cell subsets may provide an environment that allows the presence of both Th1 and Th2 cells.