The ABO system

There are four main blood groups defined by the ABO system:

  • blood group A has A antigens on the red blood cells with anti-B antibodies in the plasma
  • blood group B has B antigens with anti-A antibodies in the plasma
  • blood group O has no antigens, but both anti-A and anti-B antibodies in the plasma
  • blood group AB has both A and B antigens, but no antibodies

Almost half (48%) of the UK population has blood group O, making this the most common blood group.

Receiving blood from the wrong ABO group can be life threatening.For example, the anti-A antibodies in a recipient with group B blood will attack the group A cells if transfused to them. This is why group A blood must never be given to a group B person.

As group O red blood cells don't have any A or B antigens, it can safely be given to any other group.

The NHS Blood and Transplant website has more information about the different blood groups .

Content supplied by the NHS Website

Medically Reviewed by a doctor on 21 Dec 2018