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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I’ll get a little more specific than the other answers,, but still keep it ELI5 level.
Your body is composed of cells. Cells are little things with walls on the outside, and various things on the inside that lets them do what they need to do.
One part of the inside of most cells is the nucleus. This is where all of the instructions to make or run the cell exists.
Your body has an immune system. This is a system that attacks foreign things that get into the body that might make you sick.
But the immune system shouldn’t attack your own cells, so how does it tell the difference?
In a cell with a nucleus the cell is constantly making new things on the inside that help run the cell and the body. To make sure that the stuff on the inside is recognized as good, the cell has a little special machine or gate on the wall that grabs stuff from the inside, and shows it to the immune system on the outside. If the immune system doesn’t recognize the stuff on the inside, or if the gate itself has a funny shape, the immune system will attack and kill the cell – it’s an invader, or it’s broken in some way.
These gates have many different shapes, hundreds of them, but each person only had six shapes, so if you take someone else’s cells and put them in your body, chances are they will not match, and the immune system will kill them.
Red blood cells do not have a nucleus, and thus do not participate in this process. That’s why a blood transfusion is simpler than an organ transplant. The organ transplant has to be matched with at least 4 of the 6 gates (the gates are called _human leukocyte antigens) otherwise , even with medications, the body will attack the transplant.
There are.other ways the body protects against foreign materials, or course, so the blood type has to be correct as well.
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