eli5 why organs like liver, kidney, heart, lung require rejection meds, but blood transfusions apparently don’t.

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eli5 why organs like liver, kidney, heart, lung require rejection meds, but blood transfusions apparently don’t.

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Your immune system is designed to recognize foreign microbes and fight them. It’s how you keep from getting sick. Organs are pretty complex and can have all sorts of different microbes in them, so there’s a lot that your immune system could potentially get into a fight with.

Blood’s a lot simpler. Humans only have a few different types. Your immune system actually **would** fight the blood if you were given the wrong type – because it would have antigens that your immune system would try to destroy.

* Putting some stuff in your bloodstream doesn’t trigger your immune system as much as putting a whole organ in your body.

* There are waaaaay more potential antigens for your body to freak out about in a liver than in some blood prepped for transfusion

* Blood cells don’t last very long, while an organ (hopefully) does.

Blood transfusion rejections can happen. They’re just pretty rare while rejections are basically certain for an organ for the reasons above.

**Quick!** I need five pints of Type-O Negative blood!

That’s not too hard to come by. Oh good. And since we have the right blood type lying around, we don’t need anti rejection meds.

~ ~ ~ ~ *elsewhere* ~ ~ ~ ~

**Quick!** I need a liver from a guy who had Type-O Negative blood!

Oh, man. I hate to tell you this. There’s only three livers available right now on the whole east coast, and none of them were Type-O Negative. I mean, we could give one to you .. but you’d need anti rejection meds.

Blood transfusions are usually not meant to last. They are given if the patient is low on blood and is struggling to get oxygen to their organs. In such a situation, it’s about immediate action, and the blood transplant works fine to replace the lost blood while the doctors finish the surgery or find where they’re bleeding from. It doesn’t matter so much if the cells are killed in the next two weeks as they’ve fulfilled their purpose. They’re kind of suicide helpers as it were.
The other main role of a blood transfusion is more providing a bunch of signalling chemicals that promote tissue healing. Again, not so interested in the long term effects. Plus, red blood cells don’t have very long lives as it is, so even without being rejected, they die after a couple of weeks tops. The aim is not to permanently be a part of the patients body, but just to keep them alive long enough for them to make their own blood again.

Organ transplant you want the thing to last as long as possible as it’s hard to find a close enough match to begin with. Once you have one, you want to keep it in as good condition as you can, so you give immune suppressant medication to prevent it.

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.