system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When an organ is replaced with someone else's, the body needs immunosuppressants because of the foreign object now in the body. Why is the same not true for a blood transfusion?

Basically title! I was wondering about this. Thanks!
gpt
Blood cells don't live very long (~120 days).  The transfused ones live less time than "new" ones anyway, because of the time they lived in the donor.  The idea is to keep you alive long enough for your body to make some new blood cells that match.  The immune response kills a few along the way, it's no big deal.

Transplant a kidney, and the concept is that it will continue to function for the rest of your life.  That means it's not OK for the immune system to kill it off a little at a time.