Considering the high plasticity of these mechanisms, illustrated by the high frequency of T cells reacting to MHC-mismatched allogeneic APCs in vitro (1–10%) [28, 29], they may be responsible for the high incidence of GVHD when transplanting across multiple MHC mismatches (with growing numbers of mismatched loci increasing the probability of allogeneic reactions). Here, HLA-C is linked to graft versus host disease.