Tissue Typing

E VERYONE HAS SEVERAL ANTIGENS LOCATED ON THE SURFACE OF HIS / HER LEUKOCYTES : One particular group of these antigens is called the HLA (Human Leukocyte Antigens).

T HE HLA Is responsible for stimulating the immune response to recognize tissue as self versus non-self. Is controlled by a set of genes located next to each other on chromosome 6 called the Major Histocompatibility Complex (MHC).

The test that determines which HLA antigens are present is called tissue typing or HLA typing. Tissue typing identifies the similarity of the antigens present in both the donor and the recipient.

The closer the HLA antigens on the transplanted organ match the recipient, the more likely that the recipient’s body will not reject the transplant. For this reason, tissue typing of the kidney donor and recipient is necessary before a kidney transplantation.

T HERE ARE TWO MAIN CLASSES OF HLA ANTIGENS : Class I (HLA-A, HLA-B, and HLA-Cw) Class II (HLA-DR, HLA-DQ, and HLA-DP)

Every person inherits each of the following antigens from each parent: HLA-A antigen HLA-B antigen HLA-Cw antigen HLA-DR antigen HLA-DQ antigen and HLA-DP antigen

The set of HLA antigens received from a parent is called a haplotype. There are a variety of alleles for each of these HLA antigens.

The large number of possible variations and combinations of HLA antigens make finding a match in a family more likely than finding a match in the general public.

When performing an HLA typing test for a kidney transplant, the following HLA antigens are looked at: HLA-A HLA-B HLA-DR

The MHC genes are the most polymorphic known. There are hundreds of known alleles for each HLA Antigen. Each allele is identified by a number (i.e. HLA-A1 or HLA- A2).

Six HLA antigens are looked at for each person. Remember each person has two of each of the antigens (one inherited from the mother and one inherited from the father).

By analyzing which six of these HLA-antigens both the donor and recipient have, scientists are able to determine the closeness of tissue matching. A six-antigen match is the best compatibility between a donor and recipient. This match occurs 25% of the time between siblings who have the same mother and father.

HLA T YPING T ECHNIQUES Traditionally, HLA typing was done using serological techniques: Blood from the patient was mixed with serum containing known antibodies to determine which antigens were present. HLA typing now is predominantly done using molecular techniques: Patient’s DNA is isolated. PCR is used to amplify specific HLA genes. Genes are sequenced to determine which alleles are present.

Once the donor and recipient have been tested for tissue compatibility, the next step is an Antibody Screening ( also called a Panel Reactive Antibody or PRA). A small amount of the organ recipient’s serum is mixed with cells from 60 different individuals (each test is done separately).

P URPOSE OF A NTIBODY S CREENING Scientists can determine how many different HLA antibodies a patient has in his/her blood. If a patient reacts with 30/60 cells, he/she is said to have 50 Percent Reactive Antibody (also known as PRA). The lower a person’s PRA, the less likely he/she is to reject a transplant.

C ROSSMATCH T EST After tissue typing and antibody screening are complete and a potential donor has been identified, the final test is called a crossmatch test. Crossmatch Test: A small amount of the potential donor’s white cells is mixed with a small amount of the recipient’s serum. By exposing the donor’s HLA to the recipient’s serum, scientists can determine if the recipient has antibodies to any of the donor’s HLA.

Positive Crossmatch: A reaction between the donor’s and recipient’s samples occurs. Indicates that the recipient’s body will likely reject the implanted kidney. Indicates the transplant cannot be performed. Negative Crossmatch: No reaction between the donor’s and recipient’s samples occurs. Indicates that the recipient’s body will most likely not reject the implanted kidney. Indicates the transplant can be performed.