Tissue typing is the name given to the test that identifies an individual's Human Leukocyte Antigens (HLA). HLA's are a set of six antigens that define "self". The concepts of "self" and "nonself" explain how lymphocytes tell the difference between what to attack and what to ignore. These antigens appear on the white blood cells as well as cells of almost all other tissues. They are analogous to red blood cell antigens, types A, B, O, etc. This test is used to match a blood or bone marrow donor to a recipient. By typing for HLA antigens, donors and recipients can be matched to ensure good performance and survival of transfused and transplanted cells. A perfect HLA match occurs only between identical twins.

HLA's are inherited from our parents, and it is therefore possible to determine which set was inherited from which parent (if tissue typing has been performed on both parents). The following example illustrates how HLA's are inherited:

Antigens are inherited from each parent as a group, and each set of antigens is called a haplotype. One haplotype must be inherited from each parent. Each number represents a separate inherited antigen. Other possible combinations arising from the parents above are, (1,8,10/10,16,8); (2,7,11/3,14,17); and (3,14,17/10,16,8).

Should two siblings have exactly the same HLA, it is referred to as an "identical match" Siblings who share one-half of the same HLA are referred to as a "one-haplotype match". Siblings who do not share any of the same HLA are referred to as a "two-haplotype mismatch". There are many Human Leukocyte Antigens in the general population, and for this reason, you may share HLA with someone who is not even related to you.

After tissue typing is completed and a potential donor is identified, a crossmatch test is performed to determine if there is specific immune reactivity between the donor and the recipient. If the patient has antibodies which react to the donor's HLA's, the donor's cells will be injured. This is referred to as a "positive crossmatch" and is a contraindication to transplant. A negative crossmatch indicates that the patient does not have antibodies against the donor's HLA, and a transplant can be performed. This type of crossmatch should not be confused with the traditional crossmatch used to test the compatibility of donor red blood cells and the patient's naturally occurring antibodies (e.g. anti-A, anti-B).

Lastly, a test called "antiboody screening" determines whether or not the patient has antibodies to other Human Leukocyte Antigens. This enables the avoidance of those antigens when selecting an appropriate donor. Again, this antibody screening is not the only antibody screen that can be performed. For example, prior to any blood transfusion, a patient's blood will be tested to see if there are any unusual antibodies that would interfere with the donor red blood cells.

Integrating information from tissue typing, crossmatching, and antibody screening is extremely valuable in predicting compatibility between the recipient and the potential donor.