Blood consists of 4 parts, these are red blood cells, white blood cells, platelets and plasma. On the surface of the red blood cells are antigens which are markers. It is these antigens which are used to determine the different blood groups that exist. There are also antibodies in the blood plasma, these are the bodies natural defence system which fight against foreign antigens
Blood group A has A antigens on the red blood cells and has anti-B antibodies
Blood group B has B antigens on the red blood cells and anti-A antibodies
Blood Group AB has a mixture of both but has no antibodies
Blood Group O has neither A or B antigens but has both anti-A and anti-B antibodies.
The importance of all this is that not all blood groups are compatible and if blood from the wrong group is given to a patient the antibodies will attack cells and it could have fatal consequences.
There is also another antigen which is the Rhesus antigen (Rh), it is present in some people and not in others. When the antigen is present the blood is labelled as Rhesus positive (sometimes written as RhD+) , if it is not its labelled as Rhesus negative (or RhD-). The blood tends to be labelled firstly with the blood group A, B, AB or O then with the RhD + or -. Therefore if someone is blood group B and RhD positive they would be classified as B+ (or B positive).
In the UK 84% of the population are Rhesus positive. Blood from people who are O + is very much in demand, this is because their blood can be given to anyone who is Rh + regardless of which individual blood group they are from, and in the UK that totals 83% of the population.
The ABO method of blood grouping was first used by Nobel prize winning Dr Karl Landsteiner who discovered that there were different types of blood and also discovered the Rhesus factor.
Your blood type is inherited, one from your mother and one from your father. Both A and B are dominant genes. Therefore if you inherited A and O your blood group would be Blood Group A. If you inherit two O genes your blood group would be Blood Group O. If you inherited genes A and B then your blood group would be Blood Group AB as both the genes are dominant.
Because blood groups are genetically inherited certain blood types are more common in particular ethnic groups or in different countries. In the UK the blood group O is the most common where as in parts of Scandinavia Blood group A is most common and in Asian communities it is Blood Group B which is most common.