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.