The most studied antigen of the main human histocompatibility complex, responsible for many autoimmune reactions, is the HLA-B27 antigen. What it is, what pathologies this gene is associated with, why genetic typing is used for the presence of this antigen, we will describe in this article.
MHC and HLA
Histological compatibility is the interchangeability of tissues and cells of different people, which is important, first of all, in operations of organ and tissue transplantation. Until the 1950s, when transplanting, doctors focused on the system of compatibility of the donor and patient (recipient, recipient) for blood antigens (ABO and Rh). This method gave good results for blood transfusion, but still was not effective enough for transplantation of tissues and organs.
Since the rejection of foreign tissue is carried out using the immune system, scientists have studied the genetic similarity between people. This system is called the main histocompatibility complex (English Major Histocompatibility Complex, MHC), which is associated with white blood cells - blood cells that provide immune defense to the body. In humans, this complex is encoded by genes located in the 6th pair of chromosomes and is historically called the human leukocyte antigen (English Human Leucocyte Antigen, HLA).
Recall that an antigen is a biologically active substance of a protein nature that stimulates the formation of antibodies by cells of the immune system. Identification of histocompatibility genes is especially important during transplantation and is carried out using typing.
Unique code
The system of leukocyte antigens is individual in each person and represents various types of protein molecules that are on the surface of cells.
Human MHC is represented by two classes:
- The first class includes HLA molecules of type A, B and C. They are on the surface of absolutely all cells of the body. About 60 gene variants are known for HLA-A, 136 for HLA-B, 38 for HLA-C.
- The second class includes HLA molecules of the type DQ, DP and DR. These proteins are located on the surface of lymphocytes and macrophages of the immune system. For transplantation, HLA-DR antigen compatibility is most important.
Some general knowledge
The system of molecular antigens HLA is (that B27 is just one of a large number, we already understand) information about protein molecules that are located on cell membranes and perform the function of recognizing their own or foreign antigens and coordinate the work of the immune system. The entire system of these genes is located in the short arm of the 6th chromosome and has many alleles (variants).
The higher the degree of similarity of people in the HLA system, the higher the probability of success during tissue transplantation and the lower the likelihood of their rejection by the patient’s immune system.
But for the human body, the HLA system is that it is designed to ensure the transmission of antigens for recognition by the cells of the immune system (mainly T-lymphocytes) and to maintain the overall immune status of the body.
Typing of histocompatibility antigens
The definition of an individual HLA set is called typing. This is not a mass analysis and is carried out only in the following cases:
- When assessing the risks of developing certain genetically predisposed diseases.
- When finding out the causes of infertility and miscarriage.
- In determining the immunological compatibility of the donor and recipient.
When typing, 2 methods are used to determine the HLA variant:
- Serological (lat. Serum - "serum") method - the search for protein antigens on the surface of leukocytes.
- Polymerase chain reaction (PCR) method - using a molecular genetic study, a comparison is made of the HLA variant of the patient and the control variant for the presence of certain histocompatibility genes.
In both the first and second cases, the patient’s blood is needed for the analysis. In modern diagnostics, a HLA-B27 blood test is the most common. And that's why.
Histocompatibility Gene No. B27
Today there is a hypothesis that HLA B27 is an antigen that makes the joint tissues of carrier people similar to some antigens of infectious pathogens, namely Klebsiella pneumonia, Salmonella, Yersinia, Shigella, Chlamydia.
In the case of a bacterial infection, the human immune system begins to produce antibodies to the pathogen antigens. But in HLA-B27 carriers, after the pathogen is destroyed, the antibodies begin an attack on their own joints, which leads to the development of pathologies (arthritis).
Related pathologies
This antigen is found only in 8% of the Caucasian race (in representatives of Caucasian nationalities is more common). And yet, that HLA-B27 is a mandatory development of an autoimmune response in a carrier is not entirely true. But this antigen significantly increases the likelihood of developing the following pathological conditions:
- Ankylosing spondylitis, or ankylosing spondylitis. Antigen is found in 95% of patients. This autoimmune disease with a hereditary predisposition is characterized by multiple inflammation in the joints.
- Reiter's syndrome is secondary arthritis with inflammation of the urinary canal and mucous membranes of the eye. The presence of antigen is detected in 85% of patients.
- Secondary arthritis after genitourinary and intestinal infections. Antigen is found in 36-100% of patients suffering from this autoimmune-allergic pathology.
- Arthritis in psoriasis develops in 54% of patients.
- Arthritis associated with intestinal lesions accompanies up to 50% of patients. These are joint inflammations associated with Crohn's disease, although symptoms and treatment in adults of this pathology are considered rare, but require a more detailed description.
Diagnostic Results
If HLA-B27 was not detected during typing, then the above pathologies are unlikely. Although it is impossible to completely exclude them.
If HLA-B27 is detected, then a person should pay special attention to bacterial intestinal infections, genital tract infections. They must be treated with caution and treated on time.
It is important to remember that the presence of HLA-B27 in the genotype of an individual only gives reason to assume a predisposition to the development of joint pathologies. But in the presence of symptoms of autoimmune diseases, such typing can confirm the diagnosis.
Crohn's disease
In adults, the symptoms and treatment of this disease are largely similar to ordinary intestinal infections. That is why so often they are treated superficially.
Ulcerative colitis and Crohn's disease are more common in developed countries. Numerous studies prove the relationship between the carriage of HLA-B27 and the development of these intestinal pathologies, which are often confused due to the complexity of the diagnosis of Crohn's disease. Nevertheless, this less autoimmune pathology, when immune cells attack the intestinal mucosa, occurs in 300 cases per 100 thousand of the population.
This inflammatory bowel disease affects all parts of the digestive tract. People of both sexes are more often sick, the first symptoms of colitis appear between the ages of 20 and 40. The causes of this autoimmune disease are not exactly established, but there is a correlation with a hereditary predisposition.
Symptoms are common for intestinal pathologies. But subsequently extraintestinal symptoms are added to it: joint pain, decreased vision, rashes and ulceration on the mucous membranes.
Typing for HLA-B27 antigens, which the reader already understands, is recommended for those patients in whom colitis becomes protracted and chronic.