What is a blood type?

Blood types are determined by inherited markers, called antigens, that sit on the surface of red blood cells. The most important system is ABO, which gives four main groups: A, B, AB, and O, depending on which antigens are present. A second key system is the Rh factor, which makes blood either positive or negative depending on whether the RhD antigen is present. These combine to give the familiar eight common types, such as O positive and AB negative. Blood type matters greatly for transfusions, because giving someone the wrong type can cause the immune system to attack the transfused cells, which can be fatal. People with type O negative are known as universal donors for red cells, while AB positive individuals are universal recipients. Blood type is inherited from your parents and stays the same for life.