Why does our body reject other people’s donated organs and require immunosuppressants to be taken but getting someone else’s blood is ok?
Someone’s blood has to be matched to your blood type, else body will reject that too like a foreign donated organ.
Blood is a lot simpler and has a lot fewer antigens – that is, things that can trigger your immune system to attack. Blood is *relatively* simple, compared to all of the proteins and tissues things found in an organ.
That said, blood still has *some* antigens that your body *will* eventually notice and attack. It doesn’t matter much, though, because blood doesn’t last very long in your own body anyway. Your own red blood cells only live for about 120 days, after which they are broken down and replaced. A blood transfusion is never meant to be a permanent solution, just a temporary fix to prevent your body from dying before it has a chance to produce its own blood.
As long as the blood matches your blood type, it will take a while for your body to reject and attack the blood. In the mean time, your body will be producing its own blood so by the time the donor blood is destroyed you won’t need it.
Compare that to an organ donation where you almost certainly need that organ to continue living and can’t make your own replacement. You can’t let your donor heart get attacked and destroyed because your body isn’t going to spontaneously generate a new one and you’ll die without one.
If someone replaces your fridge, you’re going to notice. If someone replaces a plank in your floor with another plank of the same type, you probably won’t notice a thing.
Unlike other cells in your body, red blood cells do not have a nucleus, and do not have DNA. They do not synthesize RNA and do not engage in synthesis of proteins. They have no ribosomes and no mitochondria (and so no mitochondrial DNA) either.
In other words, a red blood cell from one person is pretty much functionally identical to a red blood cell from another person, provided they are the same blood type. They don’t keep pumping out a lot of the mismatched stuff that an organ would over time, and that your immune system would identify and attack as foreign.
Other blood cells like white blood cells (leukocytes) do have their DNA and do get rejected, but these generally don’t live very long already, and are not the “point” of a blood transfusion to begin with. So their loss is not particularly noticed.
Your body can reject blood too if it’s not the right type, fatally so. Beyond that the primary difference is the donated blood isn’t in your body long term, the red blood cells last up to 2 weeks and only make up a portion of your blood. Meanwhile the donor organ is meant to last for years so your body has a lot longer to reject it.