Answer:
The universal donor is blood type O. O can be injected into any other blood type because of the way blood works. Basically there are certain types of protein that cover the outsides of the red blood cells that compose blood. If someone has the A protein, he has type A and same with Type B. However, type AB has both types of protein on its "coat." Lastly, O has no protein attached to its cells and can "get along" with any other blood type. Another interesting point is that the blood type AB is considered the universal recipient because it can receive any kind of blood.