system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a universal bone marrow donor type?

There is a very rare blood type called Rh-null that basically all people can receive without having a bad reaction to it. Only 43 people have been reported to have it. Is there something similar for bone marrow?
gpt
What makes people good bone marrow donors is that their cell surface markers that the immune system recognizes (MHCs or major histocompatibility complexes) are similar enough to the recipients that the recipients don't have to be on immune suppressants for much longer stretches of time for their body to not reject the transplant or for the recipient to not have really bad graft-versus-host disease (GVHD).

Some cells downregulate MHC expression if they are cancer cells or infected with certain viruses to evade the immune response. The drawback to turning off the MHC genes is that if those cells become cancerous or are infected by a pathogen, then the immune system won't be able to recognize and kill off those cells to prevent the cancer or infection from spreading.