Class I and class II HLA antigen groups are the two most common types. Everyone has a unique set of HLA markers, and to be matched, the majority of the donor’s markers must match the recipient’s. However, because there are so many different combinations of these markers, not everyone will find a match. Depending on the rarity of the individual’s HLA antigens, the chances of a donor matching the patient range from 1 in 450 to 1 in 750,000. The greater the match, the more HLAs the donor and recipient share. In the case of graft versus host disease (GvHD), the HLA system is crucial.