Th1 and Th2 cells can cross-regulate each other, thereby suppressing/ enhancing each other’s effects.[4] Lymphocytes from subjects who are able to limit their infection to the protozoan parasite Leishmania exhibit a predominant Th1 response upon stimulation with Leishmania antigens in vitro.[25, 26] Patients with active VL, in contrast, display an impaired Th1 response,[26, 27] whereas Th2 responses, in particular IL-10 production, have been reported to increase.[28, 29] This type of response to infection often results in disseminated disease and increased mortality. This evidence concerns the gene IL10 and infection.