The cells involved in GVHD pathophysiology include CD4+ T cells; for maintaining the expansion of CD8+ T cells that mediate GVHD, APCs; these have role in the initiation phase of acute GVHD as described in murine models and Natural killer (NK) cells; contribute to tissue damage in the effector phase by releasing inflammatory cytokines and nitric oxide. However, natural killer cells mediate cell death by two important pathways: Fas-Fas-ligand-mediated apoptosis and perforin-granzyme-B-mediated cytolysis.