Why does our body reject other people’s donated organs and require immunosuppressants to be taken but getting someone else’s blood is ok?
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To put it very simple:
Your body has an immune system designed to attack anything that it doesn’t recognize as you.
This is useful when dealing with foreign invaders such as viruses, bacteria and even bigger things like worms and also your own cells if they look like they may be in danger of being becoming a cancer.
Your immune system will attack anything that doesn’t look like it belongs in your body.
This is a very useful feature and will keep you alive normally.
Occasionally it may get overenthusiastic and overdo things and attack healthy cells and you get one of those weird autoimmun diseases like Lupus.
There is also some very clever stuff build in to not attack foreign living things on the rare occasions when we want them to grow in a human body like pregnancy and to a lesser degree you helpful gut bacteria. This only rarely goes horribly wrong.
What our immune system is not designed at all to deal with it having a foreign organ transplanted into a body. Evolution has not at all prepared our biology for such Frankenstein stuff.
Not knowing any better your immune system will simply recognize what it find as a foreign invader and attack.
The immune system is not designed to recognize the foreign organ as anything other that a threat and will attack like it would attack a parasite it finds in your body.
The trick is to make it less recognizable foreign by taking an organ of someone who is either closely related to you or who just happens to be very similar to you by chance. You will still need to take meds that make the immune system less likely to attack anything at all.
Blood is easier than most other stuff because there isn’t really much variety.
There are only a very small number of different combinations of markers in your blood that might reveal it as foreign. This means that many people will share either the same set of markers as you or have one that is compatible.
If you are O- you can donate blood to everyone and it will be accepted but only receive blood from another O- person. If you are AB+ you can only donate to other AB+ persons and have everyone else reject your blood but receive blood from everyone.
There simply are too few combinations for everyone to be completely incompatible with everyone else but also there are enough combinations that transfusing blood at random can be like Russian roulette with an unknown number of bullets in the drum.
We could probably have evolved a much greater variety of blood types but that could pose a problems during pregnancy/birth when you want to avoid immune system overreacting about someone else’s blood.
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