system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 why organs like liver, kidney, heart, lung require rejection meds, but blood transfusions apparently don't.
gpt
Your white blood cells don't have eyes, so they use a secret handshake with your body to recognize all the bits that are your body (that way, they know they're friends and don't attack it). The jargon for this is your HLA type.

When you're looking for an organ donor, you want someone whose immune system has a similar secret handshake (HLA/tissue compatibility). It's nearly impossible to find an exact matching handshake (it's really complex), so doctors go with "close enough".  Otherwise, you'd never get a new organ. 

Because it's not an exact match, doctors have to give you medicine to make your immune system chill out (immunosuppressants) so that it doesn't attack the transplanted organ for having a slightly incorrect handshake. 

Red blood cells don't even have "hands", so they can't do a wrong handshake. As such, they don't get rejected by the immune system for that. They do have ABO/rH antigens (and a bunch of minor ones), and those can cause other problems, but in a completely different way. That's why blood typing is a thing.