4. The intensity of graft-versus-host reactivity of fetal lymphocytes appears to be less than that of adult cells and consequently fetal lymphocytes are more tolerant of HLA incompatibility. Published studies have shown that transplantation of UCB matched at 4-5/6 antigens results in a comparable incidence of GVHD to transplantation of unrelated stem cells fully matched at 6/6 antigens.