A histocompatibility (histo) lab, also known as a tissue typing lab, is a critical component of successful transplantation. Histo labs test potential donors and recipients for tissue compatibility. If a donor and a recipient are histocompatible, the likelihood of a transplanted organ being rejected is decreased.

"Histocompatibility" is the condition in which a donor and recipient share antigens so that a graft (donated tissue) is accepted and remains functional. Histocompatibility also refers to the study of factors that determine the acceptance or rejection of grafted tissues or organs.

The HLA System

Histocompatibility antigens, called HLA (for human leukocyte antigens), are proteins on the surface of the cells in the body. Their main function is to help the immune system defend against invaders such as bacteria, viruses, and parasites. The immune system can also recognize as foreign the histocompatibility antigens of other people's cells and will fight them, causing rejection of grafts. There are many different HLA antigens, but the ones that seem to be most important for transplantation are HLA-A, HLA-B, and HLA-DR.

Matching. Because HLA antigens can be recognized as foreign by another person's immune system, transplant professionals try to match as many of the HLA antigens as possible, between the donated organ and the recipient. That way, there is less of a chance that the recipient's body will reject the organ. In order to do this, the HLA type of every potential organ recipient is determined before they are placed on the waiting list. When a potential organ donor becomes available, the donor's HLA type is determined as well. A match program is run through UNOS and the best possible recipient for each organ is chosen. Further tests, known as crossmatches, are performed to make absolutely sure that the donor organ is suitable for the recipient.

HLA typing is performed in laboratories by trained histocompatibility technologists. It is typically performed using blood drawn from a person's arm. Using various laboratory techniques, the blood can be separated into components such as red blood cells, white blood cells, platelets, plasma, and serum. HLA typing is performed on a fraction of the white blood cells known as lymphocytes. Within the last few years, techniques using DNA for HLA typing have evolved. These techniques have enabled us to discriminate even more of the different forms of each HLA antigen.

Crossmatching is performed using serum from the recipient which is reacted against lymphocytes from the donor.

Each organ has different requirements for HLA antigen matching. For instance, kidneys are sensitive to graft rejection, so every effort is made to transplant the best possible matches. On the other hand, livers are less sensitive to rejection, so HLA matching is not as high a priority. Hearts and lungs fall somewhere between, and the effects of matching on pancreas and small bowel transplants are still being studied. Studies are also ongoing to determine which of the HLA antigens are the most important to match, which other antigens play a role, and what other factors affect organ transplant success.

Hahn, Amy B.  "Histocompatibility Laboratories Work for the Benefit of Patients."  American Society for Histocompatibility and Immunogenetics. http://www.ashi-hla.org/aboutfiles/about-patient_info.html#patients. Copyright ©1997-2002