What does ‘matching’ mean?

How well ‘matched’ a donor kidney is for a recipient is important, but what does this mean? Staff may talk about tissue type, matching and antibodies – this can be complex, but the general concepts are explained below.

Tissue Type

Your tissue type can be thought of as your barcode. It is a series of proteins on the surface of all your cells, and it is unique to you. While other people may have some proteins ‘lines on the barcode’ that are the same as you, nobody has the same combination of lines – unless you have an identical twin! Your barcode is known medically as your HLA type.

Your immune system acts like security guards. The immune cells travel around your system scanning all the barcodes. If they find one that isn’t the same as you (i.e. ‘foreign’) then it will attack and destroy it. One of the ways it does this is by making antibodies to the ‘foreign’ parts. The antibodies can be thought of as the ‘bouncers’ who get rid of the undesirable cells!

This system generally is really good, it keeps us from dying when we get infected with a bacteria or virus for example. However, in the case of transplantation, when a person gets a whole bundle of ‘foreign’ cells with a different barcode, it clearly can be a big problem. When the immune system recognises and attacks the new organ it is called rejection.

Obviously therefore the more lines on a donor’s barcode that are the same as the recipient barcode the better, as the recipient’s immune system will react less strongly as there is less that is ‘foreign’.


There are three main barcode lines (called HLA-A, HLA-B, and HLA-DR) that are most important in terms of rejection. As with every cell, there are two copies of each (one inherited from your mum and one from your dad), so there are 6 points that can be ‘matched’ or ‘unmatched’. Sometimes the transplant team may say “you are matched at 3 out of 6” for example.

Unless a transplant is from an identical twin (which happens only very occasionally!), then the person receiving the kidney will need to take anti-rejection tablets for as long as the transplant is in place. This medication suppresses the immune system so it doesn’t react to the foreign barcode. Even people who have a kidney that is matched at all HLA-A, HLA-B and HLA-DR lines, which can be called a ‘perfect’ match can still have rejection as there are other ‘minor’ lines on the barcode which are not the same.


If your immune system has ‘seen’ a foreign barcode previously then it may be that you will have produced antibodies to some of the HLA lines. This can happen if you have been pregnant, had a previous transplant, or had a blood transfusion. If someone has antibodies currently circulating in their system and they receive a transplant with even one line that there is an antibody to, then the immune system will immediately attack and destroy the new kidney. A very important part of preparing for transplantation therefore is in regularly checking for antibodies in the person needing the kidney as level or ‘strength’ of the antibodies in the circulation may change over time.

If someone has HLA-antibodies then they cannot get a kidney from someone who has even one of the HLA lines that there are antibodies to. This can make it very difficult for some people to get a suitable kidney. Such people are sometimes termed ‘highly sensitised’ and they tend to wait much longer for a kidney offer.

As with everything else in life, everyone is different! Some people seem to make antibodies much more readily than others. Some women who have been pregnant never demonstrate any antibodies while others make a lot of antibodies. If someone makes antibodies to a new transplant, the kidney will not work for as long as it would if there were no antibodies.

How important is matching?

The short answer is, the closer the match the better. However there are many other things that are also important in determining how long a kidney transplant will work. The issues to consider if the kidney is poorly matched are:

  1. Risk of acute rejection
  • 10-15% of people transplanted in Belfast have an episode of acute rejection in the first days or weeks after they receive their kidney
  • We give an additional anti-rejection injection just before and 4-days after the transplant to those receiving a more poorly matched kidney and the rate of acute rejection has not been any higher in this group
  • It is extremely seldom that we cannot successfully treat acute rejection that occurs early after transplantation with the additional treatments that are now available


  1. Risk of chronic rejection
  • Undoubtedly kidneys that are very well matched last longer than those that are poorly matched
  • However on average a very poorly matched live donor kidney will last at least as long as a perfectly matched deceased donor kidney


  1. Risk if a second transplant is required
  • If someone who has received a kidney with a poor match (e.g. none or only 1 of the main barcode lines are the same) develops antibodies to all the foreign lines, then it will be more difficult to get a suitable match for a second kidney
  • This will be particularly detrimental if the transplant lasts a very short time. The vast majority of living donor kidney transplants work very well but, as in life not everything always goes according to plan and a small number will fail within a year.
  • It is also an important consideration if a child or young person receives a kidney transplant, as they are very likely to require subsequent transplants in their lifetime
  • However if the transplant lasts 20 years or more, which is feasible, it is very possible that there will be options available for transplantation which we haven’t even imagined yet.

The bottom line is that a good quality well-matched kidney is ideal. However, since we do not live in an ideal world, the merits of each potential transplant will be considered on an individual basis. We rarely transplant a poorly matched deceased donor kidney, but will do where the options for transplantation are extremely limited e.g. due to very high antibody levels, or where dialysis is going very poorly. We will more readily transplant a poorly matched live donor kidney where the quality of the kidney is very good, and the alternative is that the person may wait on dialysis for several years for a better match but poorer quality kidney.