What is HLA-B27? HLA-B27 positive does not mean ankylosing spondylitis

Although the HLA-B27 positivity rate in patients with ankylosing spondylitis is about 90%, it has no diagnostic specificity because healthy people can also be HLA-B27 positive. HLA-B27 negative patients can also be diagnosed with ankylosing spondylitis as long as their clinical manifestations and imaging examinations meet the diagnostic criteria for ankylosing spondylitis.

1. What is HLA-B27?

The three letters HLA are the abbreviation of human leukocyte antigen (Human Leukocyte Antigen). The main function of human leukocyte antigens is to help the body identify "self" or "non-self" components. Therefore, patients receiving organ transplants must check matching before surgery, that is, whether the human leukocyte antigens of the organ donor are compatible with the human leukocyte antigens of the organ recipient. The degree of compatibility will determine the severity of the rejection reaction. There are hundreds of human leukocyte antigens, each named with a letter and a number, and HLA-B27 antigen is just one of them.

2. Is HLA-B27 hereditary?

HLA-B27 antigen is a protein molecule that is encoded by the HLA-B27 gene in the human body. The HLA-B27 gene is located on the short arm of human chromosome 6. Both parents pass on half of their HLA to the next generation, which is the theoretical basis for forensic scientists to perform paternity testing. The HLA-B27 gene is a dominant gene. During the inheritance process, if only one of the parents is HLA-B27 positive, the probability of their next generation being HLA-B27 positive is at least 50%.

3. Will a positive HLA-B27 necessarily lead to ankylosing spondylitis?

Since the discovery of the link between HLA-B27 and ankylosing spondylitis in 1973, scientists have done considerable research on the two. In my country, more than 90% of ankylosing spondylitis patients are HLA-B27 positive, while the HLA-B27 positivity rate in the general population is about 5%. However, the overall prevalence of ankylosing spondylitis in my country is only about 0.3%. In other words, only about 6% of HLA-B27 positive people will eventually develop ankylosing spondylitis. But if one of your parents or siblings has ankylosing spondylitis and your HLA-B27 is positive, your chances of developing ankylosing spondylitis are as high as 10~30%. The above data also indirectly shows that the vast majority of HLA-B27 positive people will not suffer from ankylosing spondylitis. But if you are HLA-B27 positive and have symptoms of ankylosing spondylitis, such as lower back pain or hip pain that worsens after sitting or lying down for a long time and/or causes stiffness, then you most likely have ankylosing spondylitis.

Scientists have transferred the human HLA-B27 gene into mice. If these mice are raised in a sterile environment, they will not develop spondylitis. Spondylitis will only occur when these mice are raised in an environment with normal flora. This phenomenon strongly proves that genetic and environmental factors are jointly involved in the pathogenesis of ankylosing spondylitis. It is currently believed that infection with Chlamydia trachomatis in the urogenital tract or infection with pathogens such as Yersinia, Salmonella and Shigella may trigger the onset of ankylosing spondylitis.