Some blood groups cannot be mixed with each other. So if you have a blood transfusion (where blood is taken from one person and put into another), the blood group you receive must be compatible with your own.
For example, if you are blood group A, you cannot take blood from a person with B blood as the anti-B antibodies in your blood will fight the B antigens in the donor's blood. This causes the transfused red cells to be destroyed, which can be fatal.
Blood group O- has no ABO or RhD antigens and can be given to almost anyone. Someone with blood group AB+ can usually receive blood from any group, but AB blood can only be given to a person with blood group AB.
Blood group test
To work out your blood group, your blood is mixed with different samples of serum (blood without the clotting agent), which is already known to contain different antibodies. If the serum contains anti-B antibodies and you have B antigens on your blood it will clump together, meaning that the antibodies are fighting the antigens. If the blood does not react to any of the antibodies, it is blood group O. A series of tests with different types of serum will identify your blood group.
If you have a blood transfusion, your blood will always be mixed with a sample of the donor's blood to check for clumping, even if your blood groups are compatible. This is because there are many other types of antigens in the blood, which will not always be compatible (although this is rare).
Pregnancy
A blood group test will always be done on a pregnant woman. If she is RhD negative but the child has inherited RhD positive blood from the father, it could cause complications if left untreated.
