Histocompatibility

Compatibility of tissues for transplantation

Histocompatibility refers to the genetic similarity between the tissues of different individuals, which determines the success of organ transplantation. The concept is primarily concerned with the compatibility of antigens on the surface of cells, which are recognized by the immune system.

Overview

Histocompatibility is a critical factor in transplantation medicine. It involves the matching of human leukocyte antigens (HLA) between the donor and recipient. These antigens are proteins found on the surface of most cells in the body and play a crucial role in the immune response.

Human Leukocyte Antigen (HLA) system

The major histocompatibility complex (MHC) is a set of genes that code for these antigens. In humans, the MHC is referred to as the HLA complex. The closer the match between the HLA of the donor and recipient, the lower the risk of graft rejection.

Major Histocompatibility Complex

The MHC is divided into two main classes:

• MHC Class I molecules are present on almost all nucleated cells and are involved in the presentation of peptide antigens to cytotoxic T cells.
• MHC Class II molecules are found on antigen-presenting cells such as dendritic cells, macrophages, and B cells. They present antigens to helper T cells.

Role in Transplantation

In the context of transplantation, histocompatibility testing is performed to assess the compatibility of donor and recipient tissues. This involves HLA typing and crossmatching.

• HLA Typing: This process identifies the specific HLA antigens present in an individual. It is crucial for matching donors and recipients in kidney, liver, heart, and other organ transplants.

• Crossmatching: This test determines if the recipient has pre-existing antibodies against the donor's HLA antigens. A positive crossmatch indicates a high risk of rejection.

Graft Rejection

Graft rejection occurs when the recipient's immune system attacks the transplanted organ or tissue. It can be classified into three types:

• Hyperacute Rejection: Occurs within minutes to hours after transplantation due to pre-existing antibodies.
• Acute Rejection: Occurs days to weeks post-transplant and involves T cell-mediated immune responses.
• Chronic Rejection: Develops over months to years and is characterized by fibrosis and vascular changes in the graft.

Immunosuppression

To prevent rejection, recipients are often placed on immunosuppressive therapy. These medications reduce the activity of the immune system, allowing the transplanted organ to function properly. Common immunosuppressants include cyclosporine, tacrolimus, and mycophenolate mofetil.

Related pages

• Organ transplantation
• Immune system
• Human leukocyte antigen
• Major histocompatibility complex