There are four main blood types (A, B, AB and O), each blood type is either RhD positive or negative.

The ABO Blood Group System

If your blood type is A then you have the A antigen on your red cells.

Blood type B means you have the B antigen, while type O has neither, and type AB has both A and B antigens.

The ABO system has associated anti-A and anti-B antibodies, antibodies being the body’s natural defense against foreign antigens. These antibodies are found in the plasma.

Type A has the A antigen so it recognizes the B antigen as foreign and can make anti-B antibodies.

Similarly, type B has the B antigen and therefore recognizes the A antigen as foreign and can make anti-A antibodies.

Type AB has both the A antigen and the B antigen so this group makes no antibodies.

Type O has neither A nor B antigen so this group can be given safely to any other blood type. This is why Type O donors are known as ‘universal donors’. This type can make both anti-A and anti-B antibodies if exposed to these antigens.

Giving someone blood from the wrong ABO group could be life threatening.

For instance, the anti-A antibodies type B attack type A cells and vice versa. This is why type A blood must never be given to a type B person.

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