In aGVHD the preparative regimen causes the release of inflammatory cytokines and increases the expression of MHC antigens within the host. The MHC antigens are involved in the steps leading to T-cell activation. They contain genes that encode tissue antigens used for tissue typing and these genes lie on the short arm of chromosome 6, the HLA. Even when HLA typing appears matched, patients still develop GVHD. This is thought to be secondary to minor histocompatibility antigen differences which are expressed on the cell surface as degraded peptides bound to specific HLA molecules.[18–21] Figure 1 is an illustration of the complex mechanisms that are operational in aGVHD.