The immune response of a-GVHD occurs in two phases, (Figure 1) one afferent and the other efferent. In the afferent phase, CD4+ and CD8+ T cells react to the class I and II alloantigens of the host; this occurs on the surface of the antigen-presenting cells (APC). The exact mechanism for the formation of these alloantigens is still not well understood. The conditioning regimen seems to initiate the immune response by damaging the tissues of the host, such as the intestinal mucous membrane, liver and other organs, by inducing the liberation of cytokines, especially interleukin-1 (IL1) and tumoral necrosis factor (TNFa), as well as permitting the penetration of enteric bacterial liposaccharides. The T cells are stimulated by IL-1 and by co-stimulating signals to produce interleukin-2 (IL-2). Under influence of IL-2, the CD4+ and CD8+ T cells expand clonally. They differentiate into efferent cells, which induce the graft-versus-host response. These efferent cells are activated by co-stimulators and proinflammatory cytokines, such as interferon gamma (IFN-g) and interleukin 12 (IL-12), into efferent T helper cells 1 (including cytotoxic lymphocytes CD4+ and CD8+), which direct the graft-versus-host response. The allogeneic T cells can also transform themselves into suppressor T helper 2 cells that are antigen-specific, under the influence of the interleukins 4 and 10 (IL4 and IL10).