Organs are allocated nationally on the basis of a complex clinical formula that includes urgency of need, length of time on waiting list, blood type, organ size compatibility and tissue typing. --------------- When a cadaveric kidney becomes available, the OPO notifies UNOS, and a computer-generated list of suitable recipients is created. Suitability is initially based on two factors: Blood type. Your blood type (A, B, AB, or O) must be compatible with the donor's blood type. HLA factors. HLA stands for human leukocyte antigen, a genetic marker located on the surface of your white blood cells. You inherit a set of three antigens from your mother and three from your father. A higher number of matching antigens increases the chances that your new kidney will last for a long time. If you're selected on the basis of the first two factors, a third is evaluated: Antibodies. Your immune system may produce antibodies that act specifically against something in the donor's tissues. To see whether this is the case, a small sample of your blood will be mixed with a small sample of the donor's blood in a tube. If no reaction occurs, you should be able to accept the kidney. Your transplant team might use the term negative cross-match to describe this lack of reaction. http://kidney.niddk.nih.gov/kudiseases/pubs/transplant/index.htm#process