Human Leukocyte Antigen (HLA) testing is used to match patients and donors for stem cell transplants.
HLA types are determined by a complex inheritance pattern, as described here. There are three general groups of HLA: HLA-A, HLA-B, and HLA-DR. Within these groups, there are many different types of proteins. HLA-a contains 59 different proteins; HLA-B has 118. Because your parents each have two haplotypes of HLA and you will inherit one from each, you have a 25% chance of having a matched HLA type with a sibling.
When considering a Hematopoietic Stem Cell (HSC) transplant, it is imperative that the donor and patient have sufficiently matching HLA types to complete a successful transplant. Without a sufficient match, the patient's immune system will reject the transplant.