Stem cell transplantation is an important therapeutic option for the treatment of numerous oncohematological diseases such as bone marrow aplasia, hematologic neoplasms, and hemoglobinopathies.

The transplant can be autologous (autograft), when cells/tissues are taken from the same individual, or allogeneic (allograft), when the transplant is between two different individuals.

One of the factors that most affects the outcome of a stem cell transplant is the degree of compatibility between donor and recipient. Some protein molecules (antigens) present on the surface of most cells, including white blood cells, differ from person to person. These molecules, called HLA (Human Leucocyte Antigens), distinguish each individual from one another and are involved in distinguishing between “self” and “non-self” in order to maintain their own. Therefore, HLA molecules are involved in transplant tolerance.

Before performing a transplant, it is essential to ensure that the donor and recipient have some, if not all, of the same HLA molecules (we refer to this as full or partial compatibility). If a transplanted tissue is not HLA-compatible, the transplant will not be accepted by the recipient, resulting in what is called rejection. Rejection is when the immune system of a transplanted patient attacks the new tissue and recognizes it as foreign. One of the rejection diseases is called graft-versus-host disease (GVHD).