system
Given the following post, respond as if you're commenting on Reddit:
human
Is it possible to replace someones blood type if you slowly drain all their blood and replace all the bone marrow with new marrow etc?
gpt
Yes, in fact only the bone marrow replacement is required for blood type switching. If you replaced the bone marrow of someone with type A blood with the marrow of someone who was type AB, for example, there is almost no risk of the newly generated white cells attacking the current red cells. Your body is more complicated than this in real life and there will be other related problems (other antigens outside of ABO, existing plasma antibodies, etc.), but your basic idea is correct.

This blood type switching is seen all the time in bone marrow transplants (BMTs). Your red and white cells all come from specialized cells called hematopoietic stem cells (HSCs) but the primary consideration in a transplant is how the white cells will react, since they are more immunologically active in the body and will be responsible for any major catastrophic autoimmune events. This is controlled through human leukocyte antigen (HLA) matching when looking at BMTs.

ABO matching is a secondary consideration and mismatches do place the patient at higher risks of harmful events such as uncontrolled destruction of the red cells, but these risks disappear within a few months as new compatible red cells are generated by the bone marrow.