Human T-lymphotropic virus （HTLV） is a human， single-stranded RNA retrovirus that causes T-cell leukemia and T-cell lymphoma. The virus activates a subset of T-helper cells called Th1 cells. The result is a proliferation of Th1 cells and overproduction of Th1 related cytokines （mainly IFN-gamma and TNF-alpha）. Feedback mechanisms of these cytokines cause a suppression of the Th2 lymphocytes and a reduction of Th2 cytokine production （mainly IL-4， IL-5， IL-10 and IL-13）. The end result is a reduction in the ability of the infected host to mount an adequate immune response to invading organisms that require a predominantly Th2 dependant response （these include parasitic infections and production of mucosal and humoral antibodies）.
