Hematopoietic stem cells for transplantation may be syngeneic, allogeneic, or autologous.
Identical twins are the best possible donors of stem cells.
Only about one in 100 patients undergoing transplantation will have an identical twin. If two persons do not share the same HLA antigens, T cells taken from one person will react vigorously to the mismatched HLA molecules on the surface of the cells from the other. Such reactions are against so-called major HLA determinants. Even when two persons who are not identical twins have identical HLA types, the antigenic peptides presented by the HLA antigens will differ, triggering a response against so-called minor HLA determinants. The genes encoding HLA class I and class II antigens are tightly linked and tend to be inherited together as haplotypes with low recombination frequencies.