Anti-HLA antibodies are usually referred to as panel reactive antibody (PRA) test that sometimes is referred to as percent reactive antibody, since the result is expressed as a percentage.

How can anti-HLA antibodies cause rejection?


Hyper acute rejection

In case of a kidney , circu*lating anti-HLA class I antibodies bind to the expressed antigens on endothelial cells resulting in complement activation and cell death. This in turn attracts different immune cells (to the site) such as granulocytes and platelets and the formation of microthombi and necrosis. IgG anti - HLA and ABO antibodies have been shown to cause hyper acute rejection.

Acute Rejection
There is mounting evidence to suggest that acute rejection is caused by both cellular and humoral immune responses. HLA- specific lymphocytes, antibodies, plasma cells and C4d recovered from rejected allografts suggest that both humoral and cellular response act to induce rejection.

Anti HLA antibodies induce rejection not only by activating complement but also through antibody dependent cell-mediated cytotoxicity (ADCC)

identifyication of anti-HLA antibodies?


Complement dependent cytotoxicity (CDC)

CDC was the first technique to detect HLA specific antibodies. It employs the use of live lymphocytes to detect lymphocyte*specific antibodies by activation of comple*ment system and killing of lymphocytes. This is a widely used test, but has many drawbacks,
(i) it is limited by the cell panel used,
(ii) it depends on the quality of lymphocytes and rabbit complement,
(iii) it detects non-HLA antigens
(iv) it only detects complement fixing antibodies.
Therefore, patients cannot be tagged as sensitized on the basis of this test. Results are expressed as percentage of cells reacted with the tested serum.


Elisa


The enzyme-linked immunosorbant assay (ELISA) employs the use of HLA antigens, this removes the need for complement, live cells and excludes the non-HLA specific antibodies. There are two types of commercially available kits, one detects presence or absence of PRA, the other detects antibody specificity. PRA detected by ELISA is more sensitive than the CDC assay.

Flow Cytometry
This assay depends on the availability of a flow cytometer in the laboratory. Two different methods can be employed
(i) an in house method in which whole lymphocytes are used as the antigens
(ii) comer*cially available kits which employ beads coated with specific HLA- antigens. HLA- A, B, Cw, DR, DQ, and DP coated beads have been used and shown to be much more sensitive than the CDC method.
Advances in this methodology result in the introduction of single antigen coated beads. This method helps identifying the specificity of PRA in highly sensitized patients and improves the definition of acceptable mis*matches.