Why is it important to know your blood type if you have a blood transfusion?

your body can only take your blood type. it will attack other kinds. it can be very bad, even deadly.

problems occur because the person's body rejects and tries to attack parts of the new blood resulting in severe sometimes life threatening reactions

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If you use the wrong blood type, it will clot, which can cause a heart attack.

If you are AB, you can receive all. If you are A or B, you can receive your like letter, or O. If you have O, you can only receive O, or else clotting will happen.

Then there is the Rh factor...

Positive can receive either, but Negative will clot if not Negative, even if it's the right blood type.

What would happen if you were transfused with the wrong blood type would depend upon your blood type.  AB type would be fine with any blood product. A or B would be fine with type O.  But if A were given B or B were given A  the body would go into multiple organ failure. If O were given any other type the body would go into multiple organ failure.

As for the Rh factor- if an Rh Positive is given Rh Negative nothing would happen.  If an Rh Negative is given Rh Positive blood and has never been exposed to the Rh factor nothing will happen the first time except the body will form an Anti-D antibody. An Rh Negative can be given Rh Positive blood in an extreme emergency but only once.  If an Rh Negative has been exposed the Rh factor previously the body could
go into multiple organ failure.  

As for the unexpected antibodies Lab Guy mentioned - they also would cause problems like the Anti-D.  

For transfusion the lab does an ABO; an Rh and an antibody screen. If the antibody screen is negative the lab assigns the proper units of blood.  If the antibody screen is positive the lab does antibody panels to identify what is positive.  Once the antibody is identified the proper units are assigned that are negative for the antibody.

YOU don't have to know it.  Every unit is crossmatched against units to be transfused to check for incompatibility.

Even if you do know it, you get tested anew each time.

If you get incompatible blood, you can have a transfusion reaction, which can range from a fever to hemolysis and death.