What is the main concern when tranfusing blood from one individual to another?

Compatibility of the blood products is an important concern when transfusing blood between individuals.Getting a transfusing that does not work with your blood type could result to your immune system fighting the donated blood.

Ican only answer your first question, a machine is like a human body because it's a little similar. they both almost work like the same.

This gel or liquid can be placed under the tongue of an unconscious person. this is important because you should never pour anything down the throat of an unconscious person-- s/he could choke.