[georgyo]

Also not a biologist, but the the body actively rejects all things it considers foreign.

Consider that we have A, B, O, and AB with both positive and negative variants as the most common blood types.

Unless you're O+, you will reject the blood causing a massive immune response.

Having the something that works like blood, the body won't reject, and has a shelf life longer than actual blood is hard.

[Kubuxu]

AB+ is the universal recipient of red blood cells, O- is the universal donor. It is reverse for plasma.

The blood rejection is reaction between blood plasma (with cells produced by your immune system) and red blood cells.

[pacaro]

And in addition, and TFA alludes to this, each time a person receives a transfusion there is a chance that their immune system will generate antibodies to it. That might not impact the transfusion at the time, but can make the recipient much harder to match for in the future.

My partner has had severe anemia, and because of a prior transfusion went from being an easy match (A+) to a being a 1 in a thousand match. The last time she needed blood it took 12 hours to locate and transport suitable blood.

If you read the literature on survival rates for people with very low hemoglobin level, it's all based on post operative recovery (or otherwise) of Jehovah Witnesses