If you have blood group A then you have got the A antigen on your red cells.
Blood group B means you have the B antigen, while group O has neither, and group AB has both A and B antigens.
Group O has neither A nor B antigens on the surface of their red cells so red cells of this group can be given safely to any other group. This is why group O donors are known as 'universal red cell donors. Group O makes both anti-A and anti-B antibodies.
The ABO system has associated anti-A and anti-B antibodies, antibodies being the body’s natural defence against foreign antigens. These antibodies are found in the plasma and are unusual in that they are ‘naturally occuring’ so they don’t need to be stimulated – such as by having a transfusion or during pregnancy. This means that blood from someone with blood group A contains anti-B antibodies.
Similarly, blood from someone with blood group B contains anti-A antibodies and those with group O contain both anti-A and anti-B whilst those with group AB have neither anti-A nor anti-B.
Giving someone blood from the wrong ABO group could be life-threatening.
For instance, the anti-A antibodies in a recipient with group B blood group will attack the group A cells if this transfused to them. This is why group A blood must never be given to a group B person.
