The major histocompatibility antigens of humans belong to the HLA system. They are present on all nucleated cells but can be detected most easily on lymphocytes. Each antigen results from a gene that shares a locus on the chromosome with another gene, one paternal and one maternal (two alleles). More than 27 of these antigens have been identified. The HLA complex, located in the short arm of chromosome 6, is a major histocompatibility complex that is responsible for many important immune functions in humans.

This test determines the leukocyte antigens present on human cell surfaces. When tissue or organ transplantation is contemplated, HLA typing identifies the degree of histocompatibility between donor and recipient. By matching donors and potential recipients with compatible lymphocytes and similar HLA types, it is possible to prolong transplant survival and to reduce rejection episodes. The HLA also aids in diagnosis of parentage as well as correlation with certain disease syndromes and rheumatoid diseases, particularly ankylosing spondylitis. HLA-B27, one of the HLA antigens, is found in 90% of patients with ankylosing spondylitis. Generally, the presence of a certain HLA antigen may be associated with increased susceptibility to a specific disease; however, it does not mandate that that person will develop the disease. This test is also done before HLA-matched platelet transfusion.

• Requires clinical correlation

1. Obtain a 10- to 24-mL (two green-topped tubes) heparinized venous blood sample in three lavender-topped EDTA tubes (14 mL) or two plain red-topped tubes, 10 mL minimum, or 5 mL of clotted blood or two yellow-topped (ACD) tubes. Observe standard precautions.

2. Label the specimen with the patient’s name, date, and test(s) ordered and place in a biohazard bag for transport to the laboratory.

3. Determine the patient’s HLA type by testing the patient’s lymphocytes against a panel of defined HLA antisera directed against the currently recognized HLA antigens. The HLA antigens are identified by letter and number. When viable human lymphocytes are incubated with a known HLA cytotoxic antibody, an antigen–antibody complex is formed on a cell surface. The addition of serum that contains complement kills the cells, which are then recognized as possessing a defined HLA antigen.

1. Particular HLA antigens are associated with certain disease states:

   1. Ankylosing spondylitis (HLA-B27)

   2. Multiple sclerosis (HLA-B27 + Dw2 + A3 + B18)

   3. Sarcoidosis (HLA-B8)

   4. Psoriasis (HLA-A13 + B17)

   5. Reiter syndrome (B27)

   6. Type 1 diabetes (Bw15 + B8)

   7. Acute anterior uveitis (B27)

   8. Graves disease (B27)

   9. Juvenile RA (B27)

   10. Celiac disease (B8)

   11. Autoimmune CAH (B8)

2. Four groups of cell surface antigens (HLA-A, HLA-B, HLA-C, and HLA-D) constitute the strongest barriers to tissue transplantation.

3. In parentage determination, if a reputed father presents a phenotype (genotype completely determined by heredity; two haplotypes or gene clusters: one from father and one from mother) with no haplotype or antigen pair identical with one of the child’s, he is excluded as the supposed father. If one of the reputed father’s haplotypes (gene clusters) is the same as one of the child’s, he may be the father. The chances of his being accurately identified as the father increase in direct proportion to the rarity of the presenting haplotype in the general population. Put another way, if the haplotype is very common, there is an increased probability that another man with the same haplotype also could be the father. When the frequency of the haplotype is known, the probability that the nonexcluded man is the father can be calculated. However, the degree of certainty diminishes as the incidence of the haplotype increases.

Pretest Patient Care

1. Explain the test purpose and procedure. It is also used for postmortem testing before a kidney transplantation.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.