One of the tests you will have before you go on the waiting list is HLA typing, also called tissue typing. This test identifies certain proteins in your blood called antigens. Antigens are markers on the cells in your body, which help your body, tell the difference between self and non-self.  This allows the body to protect itself by recognizing and attacking something that does not belong to it such as bacteria or viruses. 

Your body also sees antigens on a transplanted organ that are different from its own and it sends white blood cells to attack the organ. When your body attacks the new organ, it is rejecting it.  In order to help prevent rejection, you will take certain medicines called immunosuppressants.  These are discussed in another section.

Although there are many different antigens, there are six, which have been identified as having an important role in transplantation.  They are the A, B, and DR antigens.  There are two antigens for each letter and they are identified by numbers.  So, your HLA type might look something like this:

A2, A30
B8, B70
DR3, DR8

You inherit these from your parents, three (A, B, and DR) from your mother and three (A, B, and DR) from your father.  Children born of the same parents may inherit the same combination or a different combination of antigens.  If you have brothers or sisters, there is a 25% chance that you will have inherited the same six antigens as one of them, a 50% chance of having three of the same antigens and a 25% chance of having none of the same antigens. 

Except for identical twins and some brothers and sisters, it is very rare to get an exact match between two people, especially if they are unrelated.  The chance of finding an exact match with an unrelated donor is about one in 100,000.

Although we try to match antigens as much as possible for kidney and pancreas recipients, we do transplant organs into recipients who have no antigens in common, and these patients do very well.  Some of them have never had a rejection episode.  In other cases, we have seen patients who have had an exact six-antigen match have rejection occur because there are other antigens that have not yet been identified that may play a role in rejections.  Unfortunately, there is no way of predicting who will experience rejection and it can occur at anytime. 

Crossmatching is a test done just before transplant.  A crossmatch determines if your body already has antibodies formed against the donor's antigens.  It is very important to know if you have antibodies against a possible donor, because if you are incompatible with that donor you would not be able to safely receive a transplant from him/her.