Human blood group or a blood type is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells. These antigens may be protein, carbohydrates, glycoproteins etc.
A total of 35 human blood group systems are now recognised by the International Society of Blood Transfusion (ISBT). The two most important ones are ABO and the RhD Antigens which determine someone’s blood type (A, B, AB and O with +, -, or null denoting RhD status).
In 1901, Austrian Karl Landsteiner discovered human blood group after which blood transfusion became safe.