Tissue types or HLA types, as they are also called, are found on nearly all cells in the human body. The normal function of tissue type is to help the body fight infection or anything, which is not part of the body, for example, a virus.
In kidney transplantation, the donated kidney is seen as a foreign body and the normal reaction is to reject the kidney. Immunosuppressive drugs are used to dampen down the immune system and help prevent rejection. However, a better way to prevent infection and help the kidney to function for a longer time, is to transplant a kidney which has a similar tissue type. Research has shown that, in general, the better the tissue types of the person receiving the kidney and the donated kidney are matched, the longer the kidney will survive.
Tissue types are denoted as a series of letters and numbers. For example, a person’s tissue type can be written as:
HLA-A1 A2 B7 B8 DR2 DR3
Tissue types are inherited from our parents. We inherit one A, one B, and one DR type from each parent to give a full tissue type of two A, two B and two DR parts to our tissue type.
There are about 450 different forms of A, B, and DR types. As these can combine in many different formats, this means that there are millions of different tissue types. However, some tissue types are more common than others, particularly in different racial groups. A common type seen in white people may be rarer in Black or Asian populations and vice versa.
Because of the millions of different tissue types, it is unusual for patients to receive a ‘perfectly matched’ kidney. Kidneys are selected for patients using many different criteria, including level of HLA match, time on waiting list but most importantly, the crossmatching result.
Crossmatching is the final test, which takes place immediately before transplantation and involves mixing serum (a part of the blood) from the patient and cells from the donor. Regardless of how well the kidney is matched on tissue typing, patients cannot receive a kidney that is crossmatch positive as the kidney will undergo hyperacute rejection. This causes the kidney to die within minutes of it being connected. Positive crossmatches are caused by HLA antibodies in the patient’s blood.
HLA antibodies are substances in the blood which react with tissue types that are not the patient’s own. They can be formed as a result of blood transfusions, previous transplants or pregnancies (although they do not harm foetuses). They are not harmful to the patient unless they have to undergo a transplant when it is vital that the kidneys are carefully selected to ensure that the transplanted kidney does not contain the tissue type to which the antibodies have been made. So, if a patient’s tissue type is A1 A2 B7 B8 DR3 DR4 and they have antibodies to A3 and B44, they must not have a kidney with this type. They could not, therefore, receive a kidney whose tissue type was A1 A3 B7 B8 DR3 DR4, even though this tissue type is very closely matched with the patient’s own.
Sometimes these antibodies weaken over time, but in general, it is not advisable to transplant a kidney that carries the tissue type to which the patient has antibodies, even if the crossmatch is negative. This is because, once formed, these antibodies are very quickly made again by the body which will cause the kidney to be rejected. Therefore, it is very important that your blood is tested on a regular basis (every 2-3 months is advisable) for these antibodies so they can be properly identified and this information used in careful selection of a well matched kidney.