Blood Groups


There are eight blood groups which are classified using the following two systems;

  • ABO system
  • Rhesus system

The ABO system has four main groups;

  • O Group
  • A Group
  • B Group
  • AB Group

The Rhesus system is classified as Rhesus Positive (+) and Rhesus Negative (-). 

The combination of both systems define the eight different blood groups of O-, O+, A-, A+, B-, B+, AB- and AB+.

Blood Group Compatibility

Antibodies and antigens

Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma. Your blood group is identified by antibodies and antigens in the blood. link

Antibodies are proteins found in plasma and are part of your body’s natural defences. They recognise foreign substances, such as germs, and alert your immune system, which destroys them.

Antigens are protein molecules found on the surface of red blood cells.

How to test your blood type

In order to determine your blood group, red cells are mixed with different antibody solutions. If the solution contains anti-B antibodies and you have B antigens on your cells it will clump together. You are then group B.

If the blood does not react to any of the anti-A or anti-B antibodies, it’s blood group O. A series of tests with different types of antibody can be used to identify your blood group.

When receiving a blood transfusion your blood will be tested against a sample of donor cells that contain ABO and RhD antigens. Since some antigens can trigger a patient’s immune system to attack the transfused blood, safe blood transfusions depend on careful blood typing and cross-matching. link If there is no reaction, donor blood with the same ABO and RhD type can be used. link.

Check out these cool animations on blood types produced by the American Red Cross here and see the CDC table on tests used to screen donated blood. here