Transfusion-associated graft versus host disease (TA-GvHD) is a rare complication of blood transfusion, in which the donor T lymphocytes mount an immune response against the recipient’s lymphoid tissue. Donor lymphocytes are usually identified as foreign and destroyed by the recipient’s immune system. However, in situations where the recipient is immunocompromised (inborn immunodeficiency, acquired immunodeficiency, malignancy), or when the donor is homozygous and the recipient is heterozygous for an HLA haplotype (as can occur in directed donations from first-degree relatives), the recipient’s immune system is not able to destroy the donor lymphocytes. This can result in graft versus host disease.