There are four main blood groups - however there are 30 substances on the surface of Red Blood Cells (RBCs), each of which can vary and determine a subgroup of these main types.

The main interest non-clinicians have is in the main four - A, B, AB and O. The blood type is determined by the type of antigens in the cell, A has A-type, B has B-type, AB has both A- and B-type together and O has no antigens.

The importance of this information is that in a blood transfusion where the patient must be given a compatible blood type: A can receive A- or O-type, B can receive B- and O-type, AB can receive A-, B- or O-type, O can only receive O-type.

People with O-type blood are known as universal donors, people with AB type are known as universal recipients.

Giving the wrong blood type would cause the patients antibodies to reject the new blood.

A point of interest is that pregnant mothers may have a different blood type to their unborn child, in which case the mothers immune system will build a resistance to avoid rejection of the fetus.