Blood groups, the ABO system

People may be one of four possible major blood groups. They may be A, B, AB or O. In addition to these four groups there is a further group called the Rhesus factor. This is an additional group which a person may, or may not have, in addition to being A, B, AB or O. An individual may therefore be A Rhesus positive or Rhesus negative, B positive or negative, AB positive or negative or O positive or negative. The blood group a person has depends on the nature of their red cells, which is in turn genetically programmed. Blood groups are determined by protein based structures located on the external membranes of the red cells.

These grouping molecules are specific proteins which possess a particular molecular shape and are referred to as antigens. Someone is blood group A if they have A antigens on their red cells. In blood group B the person has B antigens on the surface of their red cells. The presence of group A and B antigens on the surface of the red cells means the individual would be group AB. People with group O have no antigens on the surface of their red cells. In fact O is a corruption of zero so really the blood groups should be A, B, AB and zero but we have changed this to O. So a person with blood group O actually has no ABO grouping antigens on the surface of their red cells. The Rhesus factor is also an antigen which is simply present or absent on the surface of the red blood cells.