Blood groups

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 .

Medically Reviewed by a doctor on 21 Dec 2018

Articles for blood groups

Introduction

There are four different blood types, known as blood groups. The four main blood groups are A, B, AB and O.

What is blood?

Your body carries aroundfourto six litres (7to 10.5 pints) of blood. Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma. Plasma is about 90% water, but als

Antigens and antibodies

Your blood group is identified by antigens and antibodies in the blood. Antibodies are part of your body's natural defences against invading substances such as germs. Antigens are protein molecules f

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 ant

The Rh system

Red blood cells sometimes have another antigen, a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it's absent, your blood group is RhD negative. This means y

Blood group test

To work out your blood group, your red cells are mixed with different antibody solutions. If, for example, the solution contains anti-B antibodies and you have B antigens on your cells (you are blood

Pregnancy

Pregnant women are always given a blood group test. This is because if the mother is RhD negative but the child has inherited RhD-positive blood from the father, it could cause complications if left u

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