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...
