Knowing your blood type is important, mainly because it determines who you can donate blood to, but also who you can receive blood from.

There are four basic blood groups: A, B, AB and O. Each is defined by which molecules called antigens and antibodies are present on the surface of red blood cells. Type A blood contains A antigen and the B antibody. Type B contains the B antigen, and the A antibody and Type AB contains both – A and B antigens and no antibodies. Type O contains neither.

In addition, the Rhesus (Rh) factor determines whether your type is positive (+) or negative (-) and together with your blood type determines who you can donate blood to, and whose blood you can receive. People with blood type A, B, AB and O can receive their own blood type with +/- Rh factor. People with O-negative blood, for example, can give blood to recipients of any blood type but they can only receive blood from O-negative donors. People with O-positive blood can give blood to A, B, AB or O-positive recipients. The opposite is true for people with an AB-positive blood type, their donations can only be used for patients who are AB-positive, but they can receive blood from donors of any blood type.

The ABO blood group system is the single most important classification system in human blood transfusions. A mismatch in ABO typing can cause serious reactions and even death in the recipient.