In an early step of GVHD an accumulation of CD8 T cells and mast cells is observed in the host's epithelium; following degranulation, activated mast cells produce IL-10 to stimulate regulatory CD4 T cells which in turn may produce IL-9 for the stimulation of mast cells. In mast cell deficient mice (C57BL/6-Kit^W-sh/Wsh) regulatory CD4 T cells may not be sustained and other T cells may proliferate in an overwhelming fashion (see Lu et al⁷ ).