How many blood groups are there?

 Division into blood groups is based on inherited characteristics found in blood components. There are several blood-grouping systems. From a medical point of view, the most important are the ABO and Rhesus systems. The ABO system distinguishes four blood groups, which are known as A, B, AB and O, depending on certain distinguishing marks on the surface of the blood cells. The blood groups A and O predominate in central Europe, and only 7% of people belong to blood group AB. Division by the Rhesus system is based on other surface characteristics of the red corpuscles, Know as Rhesus factors, of which some 40 are known. In addition, there are 19 or more blood grouping systems which were mainly used in paternity testing and forensic medicine before the introduction of DNA analysis.

Because mixing blood from different groups causes defensive reactions in the body-such as particles clumping together-it is essential that a patient's blood groups is known before a blood transfusion can take place.