HLA and MHC, Graft Versus Host Disease (GVHD)
The major histocompatibility complex (MHC) relates to antigens found on all nucleated cells. It was initially designated human leukocyte antigen (HLA) system because tissue typing was primarily performed on peripheral blood lymphocytes.
The MHC system consists of a trio of class I antigens called A, B, and C that are present on all nucleated cells. The MHC class II antigens have a limited distribution (mostly on mononuclear inflammatory cells). There are DP, DQ, and DR antigens, but the clinically relevant one is DR.
Why is the MHC (HLA) system important?
The MHC system is involved in immune responses. Many infections, particularly viral infections, are controlled by CD8+ lymphocytes that destroy virus-infected cells displaying class I antigen in conjunction with viral antigen. MHC lass II antigens help to induce CD4+ lymphocytes that boost immune responses.
In order for many transplants to be successful, there must be HLA tissue typing. There are 6 major alleles-those for A, B, and DR.
MHC antigens may be associated with a variety of diseases, such as HLA B27 with ankylosing spondylitis, or HLA DR 2, DR3, and DR4 with autoimmune diseases.
Graft Versus Host Disease (GVHD)
GVHD is a situation where transfused lymphocytes engraft and multiply in immunocompromised patients (e.g., bone marrow transplant patients). The newly engrafted lymphocytes attack the host. This is the opposite of a host rejecting a transplanted organ (e.g., a heart).
Transfusion-associated graft versus host disease (TAGVHD) is a different disease from GVHD in allogeneic bone marrow transplant recipients. TAGVHD is uniformly fatal and untreatable. It occurs in immunocompromised patients when the blood products contain T-lymphocytes and attack many host tissues.
TAGHVD is prevented by gamma-irradiating the blood products to be transfused.