Blood Group is an important medical term that refers to the type of blood a person has. It is determined by the presence or absence of certain antigens on the surface of red blood cells. The most common blood group system is the ABO system, which has four main types: A, B, AB, and O. The RELS crossword solution refers to the Rhesus (Rh) factor, which is an antigen found on the surface of red blood cells. People with the Rh factor are considered Rh positive, while those without it are Rh negative.