[pharmacist2030 (OP)]

What happens if cold-blooded animal's blood is mixed with a warm-blooded animal's blood? In other words, could we donate our warm blood to cold blooded animals and vice versa?

[Kryptid]

It would almost certainly not be compatible. If we have to be careful about which humans receive what blood type during transfusions, no doubt that problem would be greatly exacerbated if we were to try to transfuse with a reptile or amphibian. The antigens on the surface of our blood cells would probably be recognized as foreign. Alternatively, reptiles probably have different oxygen and nutritional needs that our blood could not satisfy.

[chris]

The answer is no, you could not do this. Blood cells carry surface antigens, which are complex sugar molecules, that are specific to different species and even different groups of individuals within those species. Antibodies present in the blood recognise antigens on cells that are not native to an individual and trigger an agglutination reaction, which makes the blood clump, fatally.

Blood transfusions even between humans need to be conducted carefully to avoid these reactions. There is little hope that transmission between highly diverse species such as cold and warm-blooded animals would work. That said, I don't know how closely related birds (warm blooded) and reptiles are...

[Bored chemist]

"In other words, could we donate our warm blood to cold blooded animals and vice versa?"
No

Something like this may happen.
https://en.wikipedia.org/wiki/Graft-versus-host_disease
Though a straightforward massive reaction to foreign protein would be more likely.
Something like this
https://en.wikipedia.org/wiki/Serum_sickness

[chris]

Actually it's much more likely to trigger a gross agglutination reaction in the recipient, because there will be very few incoming antibodies and white cells but lots of foreign antigen. Hence the recipient immune response will "win" and bind the whole lot together into a pathological blood clot (the posh word is "thrombus").

[pharmacist2030 (OP)]

Thanks for the explanation! I have read that the main difference between the warm blooded animals and cold blooded ones is the red blood cells. Cold blooded animals red blood cells contain a nuclei which is not found in their counterparts. The other issue is that there is also a difference between warm blooded animals blood ; for instance, between Human and dogs blood. Human blood groups are A, B, AB and O; on the other hand, dogs blood is not classified as human, it is called blood Erythrocyte Antigen (DEA). This might keep it even more challenging to transfer blood between warm blooded animals. I hope in the future there will be technologies that will enable us to transfer blood between warm and cold blooded animals, definitely it will be a life-saving technique.

[chris]

More likely is that stem cells will be used to mass produce blood in vitro, avoiding many of the other problems of blood transfusion, including supply shortages and infection risks, let alone group incompatibility issues.