There are many different blood group systems, but the two most important and well-known are the ABO blood group system and the Rh blood group system.
The ABO blood group system consists of four main blood types: A, B, AB, and O. Blood type is determined by the presence or absence of antigens (A and B) on the surface of red blood cells. People with blood type A have A antigens on their red blood cells, people with blood type B have B antigens, people with blood type AB have both A and B antigens, and people with blood type O have neither A nor B antigens.
The Rh blood group system is based on the presence or absence of a single antigen called D. People who have the D antigen are Rh-positive (e.g., A+, B+, AB+, O+) and people who do not have the D antigen are Rh-negative (e.g., A-, B-, AB-, O-).
Other less well-known blood group systems include the Kell system, Duffy system, Kidd system, MNS system, and Lewis system, among others. These systems are important for certain medical conditions and may affect the compatibility of blood transfusions.
It is important to note that blood type and blood group systems can vary among individuals and populations. In addition, there are many different subtypes and variations within each blood group system that can affect blood compatibility and transfusion outcomes.