Red blood cells have various proteins on their surface called antigens. There are various types of red blood cell antigens - the ABO and rhesus types are the most important. Different combinations of these types give several different blood groups. There are four main blood groups: A, B, AB and O with each group being either RhD positive or RhD negative, resulting in a total of eight main blood groups. Your genetic make-up which you inherit from your parents determines which antigens occur on your red blood cells and therefore which blood group you are.