Blood is classified according to several factors, such as the four blood groups A, B, AB and 0, as well as the Rhesus factor. The latter refers to a protein that is found on the surface of the red blood cells. If this protein, also known as an antigen, is present, the blood is said to be Rhesus-positive. If it is missing, the blood is Rhesus-negative. This classification is important when it comes to blood donations (see box). In total, there are over 40 different blood group systems, most of which are of no importance for everyday medical practice.
