From this, we can see that matching is not for treatment, but to prevent rejection reactions and transplant failure. Simply put, if it weren't for this HLA threshold, anyone could donate hematopoietic stem cells to the patient.
HLA is a system, a leukocyte antigen system, that includes many things. The focus on HLA is because it is the primary antigen causing transplant rejection reactions, thus called transplant antigen. Is it only HLA that causes rejection reactions? Certainly not. As mentioned before, medical research can only apply what current study has reached.
Other factors could be a concern, which is why medicine first seeks donors within the family. While the chance of HLA compatibility is high, it's also possible to exclude other factors causing rejection reactions, given that they have partially similar genetic genes.
