24 September 2014 (Wednesday) - HLA B27 status determination
Human Leukocyte Antigen (HLA) B27 is a class I surface antigen encoded by the B locus in the major histocompatibility complex (MHC) on chromosome 6 and presents antigenic peptides (derived from self and non-self antigens) to T cells. According to Wikipedia (!)
Individuals with ankylosing spondylitis (AS), and other associated inflammatory diseases referred to as "spondyloarthropathies" are far more likely to have this antigen than not. In fact over 95% of sufferers have this antigen compared with a far lower incidence in the general population: about 8% of Caucasians, 4% of North Africans, 2-9% of Chinese, and 0.1-0.5% of persons of Japanese descent.
Hence the antigen is a useful marker when investigating the suspicion of ankylosing spondylitis
When testing for the presence or absence of the HLAB27 antigen two similar assays are run in parallel. In each instance two aliquots of patient (or control sample) are taken and mixed (in separate tubes) with an isotype control and with anti-HLA B27 and incubated.
A lysing agent is added, and after an incubation period the remaining white cells are washed twice with phosphate-buffered saline. It's important to spin gently; white cells are fragile things.
After the final wash the cells are re-suspended in phosphate-buffered saline and analysed on the flow cytometer. The isotype control (first tube) gives a zone on which to target the analyser for the second tube which will then exhibit positivity or negativity. And being performed in duplicate with differing antibodies gives little scope for error.
The result is positive or negative; because one either has or does not have the HLA B27 antigen.
Equivocal results are occassionally due to cross-reactivity from other antigens (HLA B7 is the most likely), but most commonly from a compromised sample. Repeating with a fresh sample will give a definitive result.
I learned a new technique today...