A set of inherited characteristics on the surface of cells. Each person’s tissue type has six components (three from each parent). Although there are only three main sorts of tissue type characteristic (called A, B and DR), each of these comes in 20 or more different versions. Given the large number of possibilities, it is unusual for there to be an exact tissue type match between a transplant kidney and its recipient. However, the more characteristics that match, the more likely is a transplant to succeed.