HLA-B27 is a small gene that is found in 4-24% of the population (depending on ethnicity).
HLA-B27 is associated with several diseases, including Ankylosing spondylitis (AS), Iritis/Uveitis, Colitis and Arthritis,
but
only a very small proportion of people with the HLA-B27 gene actually get disease.
As an example, only 3-5% of people with HLA-B27 get AS.
HLA–B27 is a gene which is part of a bigger cluster of genes called ‘the HLA’.
This cluster of genes is responsible for coding for ‘markers’ on the surface of cells that are found on all cells across the body.
To be clear, the genes code for the markers on the cells that the genes are in and ‘switched on’ in. It is not a master control system where a gene in a cell in the brain codes for the marker to be produced in a cell in the liver!!
HLA is an abbreviation that stands for ‘Human Leucocyte Antigen’.
In plain language this means:
“protein markers on the surface of cells of people that are recognised by their own white cells” [much quicker to say ‘HLA’ really….!]
There are different classes of cell markers coded for by this HLA group of genes.
The different classes code for different types of proteins, occurring in different cells on the cell walls: and being different in size and shape, they have different functions.
These proteins are in general needed to display antigens on the cell walls to give a signal to the immune system that there is a problem like an infection in the cell.
The presence of these antigens as ‘read’ by the white cells determines the fate of the cell… and its fate is sealed!
Showing these ‘antigens’ on the cell wall is like raising a ‘red flag’ to the immune system: ‘killer T cells’ receive this as a message to engulf and destroy the cell, which has indicated it is infected by displaying the ‘antigen’ on its surface.
HLA–B27 is part of a group which encodes for proteins that are in the cell membrane and have the particular function of bringing peptides (broken down bits of protein) from inside the cell to the cell surface for immune cells to see. Peptides can come from ingested bacteria or viruses.
HLA–B27 is a gene that is found in 4-24% of the normal population, varying according to race.
Of those people who have HLA–B27, only a small proportion go on to get a disease associated with the allele.
Only 3-5% of those people with HLA–B27 get Ankylosing Spondylitis.
We don’t understand how this HLA-B27 is associated with the disease, in particular with disease in so few people who carry the gene…
HLA-B27 is important in Rheumatology because of its association with Ankylosing Spondylitis and Spondyloarthropathies.