If you’ve ever given blood, received blood or even just had a blood test, you’ve probably heard something about blood types.
There are 4 main types of blood: A, B, AB and O
When one person receives blood from another (blood transfusion) it is crucial that they receive blood that is compatible with their own. 
In the diagram, you notice how O blood types can give blood to everyone, but can only receive their own type.
AB types on the other hand can receive from everyone but can only give to other ABs.
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Why Blood needs to be compatible…
Each blood type has a different combination of Antigens (substances that can cause allergic reaction in blood) and Antibodies (a protein that identifies and neutralizes substances like antigens).
If the person receiving blood has antibodies that match the donor blood’s antigens, the antibodies in the donated blood would attack those antigens by binding together in clumps.
This can cause problems…
It would clog blood vessels, potentially stopping circulation in various parts of the body.
Those clogged red blood cells (which contain hemoglobin), may also crack and leak their contents into the body. Hemoglobin is what transports oxygen from the lungs to the rest of the body. When hemoglobin goes outside of its red blood cell, it becomes toxic.
All in all, it could be fatal!
Sources: wikipedia | nobelprize.org