Blood groups - NHS Choices
Blood groups 
Blood groups
Introduction 
There are four main blood groups (types of blood): A, B, AB and O. Your blood group is determined by the genes you inherit from your parents.
Each group can be either RhD positive or RhD negative, which means your blood group can be one of the eight types shown below (see the Rh system ).
What is blood?
Your body carries around four to six litres (7 to 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 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 carry oxygen around the body and remove carbon dioxide and other waste products; they give 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 clot (thicken) to stop bleeding
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 found on the surface of red blood cells. Antibodies are proteins found in plasma. Antibodies recognise anything foreign in your body and alert your immune system to destroy it.
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.