The term ’blood group’ usually refers to an individual’s combination of Red Blood Cell (RBC) surface antigens. Antigens are specific sites on different proteins, glycoproteins or glycolipids that form parts of the RBC membrane which the immune system can interact with. These proteins have numerous functions such as: membrane transporters (Diego, Kidd), receptor and adhesion molecules (Duffy, Lutheran), complement regulatory glycoproteins (Cromer, Knops), enzymes (Yt, Kell, Dombrock), structural components (Diego, Gerbich) or components of the glycocalyx (MNS).
Antigens are defined by antibodies that occur either ‘naturally’ due to encountering antigens ubiquitous in the environment or are formed as a result of active immunisation to non-self RBC antigens following exposure to human RBCs from another individual. It is the presence and absence due to inherited variation of red cell surface antigens that defines the blood group of an individual.
Blood group systems are officially defined as systems of one or more antigens governed by a single gene or complex of two or more closely linked homologous genes. Each system is genetically discrete from every other blood group system. In order for a blood group system and its antigens to be recognised the underlying genetic variation must be identified, sequenced and confirmed to affect phenotype.
The International Society of Blood Transfusion (ISBT) Working Party for Red Cell Immunogenetics and Blood Group Terminology (ISBT WP) maintains an official record of all currently recognised blood group systems. There are currently 45 recognised blood group systems containing 360 red cell antigens (July 2023). The 45 systems are genetically determined by 50 genes. For more information please click on the following links:
ISBT also maintain three categories for antigens that have not yet been linked to blood group systems. Collections (the 200 series) were designed to group antigens which are biochemically, genetically or serologically similar where the genetic basis has not yet been discovered. There are also two antigen series; the 700 Series contains antigens which do not fit into any system or collection which have an incidence of <1% across all human ethnic populations, and the 901 Series contains high incidence antigens with an incidence of >90% and cannot be included in a system or collection.