Give Blood England and Wales

The most important blood groups in transfusion are the ABO blood group system and the RhD blood group system.

Blood groups are determined by a protein (antigen) on the surface of the red cell. So, the ABO system has A and B antigens and the RhD system has the D antigen.

In all, there are 30 major blood group systems. This means a person may be A RhD positive, and at the same time Kell (Kell system) positive, M and N (MNS system) positive and Lea and Leb (Lewis system) positive.

The ABO blood group system

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.

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.

Blood group A has the A antigen. This group recognises the B antigen as foreign and can make anti-B antibodies.

Similarly, blood group B has the B antigen and therefore recognises the A antigen as foreign and can make anti-A antibodies.

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

Group O has neither A nor B antigen so this group can be given safely to any other group. This is why Group O donors are known as "universal donors". Group O 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 in group B attack group A cells and vice versa. This is why group A blood must never be given to a group B person.

The RhD system

Another important blood group system in transfusion is the RhD system.

85% of people have the D antigen on their red blood cells and are RhD positive.

The remaining 15% lack the D antigen and are RhD negative.

Your blood group is defined by your ABO group together with your RhD group. For instance, someone who is group A and RhD negative is known as A negative.

The chart below explains the national distribution of blood groups. Click on a blood type for more information.

ABO Blood Group	Rh(D) Type	Percentage of Population with this Group
O + Pos 37% O - Neg 7% Total Blood Type O   44% A + Pos 35% A - Neg 7% Total Blood Type A   42% B + Pos 8% B - Neg 2% Total Blood Type B   10% AB + Pos 3% AB - Neg 1% Total Blood Type AB   4% Total Pos 83% Total Neg 17%