What determines our HLA typing?
We often think, wrongly, that having the same blood group and the same rh as someone waiting for a transplant is enough to be able to donate an organ. However, the reality is a little more complicated.
Like red blood cells linked to our blood group, most other cells in our body have their own “identity card”: HLA typing.
This HLA system (for Human Leucocyte Antigen) is defined by antigens, molecules linked to the immune system, present on the surface of our cells and in particular of our white blood cells. These antigens help the immune system differentiate what is in the body from what is foreign. This recognition system is called the major histocompatibility complex (MHC).
As with any molecule in the body, the antigens in our cells are determined by genes on our DNA.
The genes of the HLA system are divided into two groups. Class I genes determine antigens present on all nucleus cells in the body and are classified into three groups (A, B and C). Class II genes define the antigens of certain cells of the immune system only, and are also divided into three categories. The system is therefore very complex, but a simple blood test is enough to determine the HLA typing of a person.
The success of a transplant depends on HLA compatibility
Any cell showing the HLA typing of a given person belongs to that person. Conversely, any cell with a different HLA or without HLA will be considered an intruder to be destroyed by the immune system.
In this sense, it is essential during a transplant, that the donor has an HLA typing as close as possible to the recipient, in order to minimize the risk of rejection.
The closer the HLA typing, the greater the success of the transplant, because sometimes the anti-rejection treatment is not enough. In general, at least 6-10 antigens should be compatible, the ideal would be a compatibility of 6/6 or 10/10. A transplant can however be attempted with less compatibility (5/6 or 9/10).
As the determination of HLA typing is linked to genes, it is within the same family that we are most likely to find two HLA-compatible people. Each of the two parents passes on half of their HLA genes to their child. Thus, within a sibling, two children can be 100% compatible if they have received the same maternal and paternal genes. They can also be semi-identical HLA if they have only 50% of the genes in common, or be completely incompatible. As this compatibility is very rare between two unrelated individuals (one chance in 40,000), transplant waiting times are very long.
Note that since HLA typing comes from heredity, mutations occurring in the genes in question can be transmitted to the offspring, who can then inherit an autoimmune disease (type 1 diabetes, multiple sclerosis, lupus …).
Read also :
Blood group: which means this classification
What is the rhesus of the blood group?
Organ donation: an amendment to increase the number of available grafts
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