Tissue typing for kidney donation

There are various stages to checking the tissue type of a donated kidney and deciding how good a match it is to your child.

Note: The donor is the person giving their kidney and the recipient is the person receiving it. The donor can either be a live donor or one that has died (deceased).

The first step is to look at your child’s blood group and that of the donor.

• If your child is blood group AB, which is rare, a person with any blood group can donate.
• If your child is one of the other blood groups (A, O or B), the donor must either be the same blood group or blood group O, which can donate to all.

The donor’s blood group may not necessarily be one that is compatible with your child. If it isn’t, your child cannot receive this donated kidney. Research may find ways around this in the future, but we do not know when this could happen.

If the donor’s blood group is compatible, we can then proceed to tissue typing to see how ‘well matched’ the kidney is to your child. Contrary to what it sounds like, tissue typing only involves a simple blood test.

The tissue type is what makes someone react to a donated kidney. Normally, tissue type is involved in the process of getting rid of viruses from the body.

There are six principal pieces of protein in the cells of the body that are involved in this process. These are called HLA antigens and scientists have given them each a different number, as they have been discovered. Your child will have inherited three of the six HLA numbers from each parent, but cannot inherit a number that neither parent has.

• For example, if the mother’s HLA numbers are: 1 2 3 4 5 6 and the father’s HLA numbers are: 7 8 9 10 11 12 then their child could have HLA numbers: 1 2 3 7 8 9 or 4 5 6 10 11 12 or 2 4 5 7 9 10 but could not be 1 2 3 7 10 13 as neither parent has HLA number 13

The outcome of this is that if the donor is not genetically related to the child, such as a step mum or step dad, foster parent or adoptive parent they are unlikely to be a good match, so we would not usually consider them as a donor.

It is important for you to understand that tissue typing is similar to what is sometimes called ‘paternity testing’. That is, the results may help to confirm (or not) whether the donor is genetically related to the child.

• For instance, the results could show that the man presumed to be the child’s father is actually not related genetically to them so cannot be the biological parent.
• Similarly, if a child is conceived using donated eggs and/or sperm, they will not be genetically related to the parents.
• However, if a child is carried by a surrogate but was conceived using the mother’s own eggs or father’s own sperm, there will be a genetic relationship, even if someone else carried and gave birth to the child.

We ask that you consider this carefully and before agreeing to the test, we would want you, as a family, to decide who should be told if the results are unexpected. That is, if the tests were to show that one or other of you is not the ‘biological parent’, would both of you, one of you or neither of you want to be told?

If you have any questions or concerns about this, you must discuss them either with the doctor or nurse or your family doctor (GP) before undergoing the blood test. We ask that you consider this carefully.

We prefer a good match if possible as this reduces the likelihood of the new kidney being rejected by the recipient’s body, although this is more important for deceased donor kidneys.

Deceased donor

When a deceased donor kidney becomes available, UK Transplant matches the six numbers that the donor has with the best match for all the potential recipients on the on-call register.

One way of thinking of this is that it is very much like the National Lottery. That is, we all have our own six numbers and if the donor had the same six numbers as one of the recipients on the list, they would be offered that kidney. This is called a ‘6-antigen match’ or a ‘full house match’.

Just like winning the lottery jackpot, this does not happen very often. Five numbers matching is a bit more common, and four numbers matching is a bit more common again. However, we do not like to take less than four numbers matching as far as possible, unless the child has numbers that are extremely uncommon in the population and very unlikely, therefore, to be matched.

If the child is lucky enough to receive a full house 6-antigen matched kidney from a deceased donor, then this is the only match that does as well as a kidney from a parent.

Another reason why we prefer well-matched kidneys is that if your child meets with an HLA number that they do not have, they make an antibody to it. This means that if the transplant fails, they cannot be given that number again. This makes it more difficult to find a transplant for them the second time.

• For example, if the donor has HLA numbers: 1 2 3 4 5 6 and the recipient has: 1 2 3 7 8 9 then the recipient will make antibodies to numbers 4, 5 and 6. Therefore if the kidney fails and another donor kidney is offered then the donor cannot have the tissue typing that includes HLA numbers 4, 5 & 6 as the recipient would have antibodies that would fight against and these HLA numbers, which would result in the recipient’s body rejecting the kidney.

Some HLA numbers are more common in the population than others, so there are some numbers that we are particularly keen not to mismatch.

Live donors

A parent or older sibling can donate a kidney to the child even if their tissue match is not as good. This is because the parent or sibling may have other things in common that we do not match for or even understand. It is also because the kidney is taken from the parent/older sibling and very rapidly transferred into the child, with very little wait in between.

Is it possible to decrease the chances of recipient developing antibodies to the mismatched numbers, if they were to have a deceased donor kidney first?

What we can do is to put your child on-call for a new kidney, but only accept a kidney with your mismatched numbers excluded.

• For example, if you are: 1 2 3 4 5 6 and your child is: 1 2 3 7 8 9 then we can ask UK Transplant to put your child on-call, but not offer any kidney that has numbers 4,5 or 6.

Your child should not then make antibodies to any of the mismatched numbers but we can never guarantee this. Blood may need to be given at the time of the transplant, and as blood cells also carry these numbers and the blood may have come from several different donors, your child may make antibodies to them. Some antibodies also react with more than one HLA number.

When we have decided who will be the donor, we need to put the donor’s blood and the recipient’s blood together to be sure that they do not react. This is called cross matching. They will only react if the recipient has HLA antibodies already, for example, if they have had a previous transplant or if they have been given blood previously.

• If the bloods react against each other, this is called a ‘positive cross match’ and the transplant cannot go ahead.
• If the cross match is negative, all is well and the donor can then carry on with tests to establish that they are healthy and able to donate a kidney.