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The four main blood groups are A, B, AB and O.

Each group can be either RhD positive or RhD negative, which means that your blood group can be one of eight types.our body carries around 4-6 litres (8.5-12.5 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 also contains proteins, nutrients, hormones and waste products. Blood is made up of about 60% plasma and 40% blood cells.

Each type of blood cell has a specific role to play:

Red blood cells transport oxygen around the body and remove carbon dioxide and other waste products; they give your blood its red colour.

White blood cells are part of the immune system (the body's natural defence mechanism) and help fight infection .

Platelets help the blood to clot (thicken) to stop bleeding 

Our blood group is identified by antigens and antibodies present in the blood. Antigens and antibodies are our blood's natural defences against foreign substances.

Antigens are protein molecules found on the surface of red blood cells. Antibodies are found in plasma. Antibodies recognise anything foreign in our body and alert our immune system so that it can destroy it.

The ABO system:

Blood groups are 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.

Group AB has both A and B antigens but no antibodies.

Group O is the most common blood group in the UK, with 44% of the population having group O blood (37% O+ and 7% O-).

Receiving blood from the wrong ABO group could be life-threatening because antibodies in a person with group A blood will attack group B antigens and vice versa. This will cause a severe reaction in the person receiving the blood.

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