Blood groups are an important part of medical science. They are used to identify individuals and determine compatibility for blood transfusions. Blood groups are divided into three categories: ABO, Rh, and other rarer TYPES. The ABO system is the most common and is divided into four groups: A, B, AB, and O. Each group is further divided into two subgroups, positive and negative. The Rh system is also divided into two subgroups, positive and negative. Finally, there are a variety of rarer blood TYPES, such as the MNSs, Kell, Duffy, and Kidd systems. These are all important for determining blood compatibility and can be used to identify individuals.