An ideal 6:6 antigen match, known as a "clinical match," means that both sets of the three "most important" inherited human leukocyte antigens in the donor match perfectly with those on the body (and immune) cells in the patient. Below, only Child E would serve as a 6:6 clinically matched donor for Child A; one sibling is a total mismatch; the other siblings and both parents would be haplo-identical because only one of their two inherited haplotypes matches Child A.