Karl Landsteiner discovered the four main blood groups: A, B, AB, and O. The presence of antigens (proteins) and antibodies determines blood type. Group O blood lacks antigens and contains antibodies, making it the universal donor type. Group AB has both antigens but no antibodies, so it can receive blood from any group but can only donate to other AB types. A person's Rh factor is also important, as Rh negative blood can only be received by other Rh negative recipients. Different animals have blood that ranges in color from red to green, yellow, white, and blue due to varying respiratory pigments and compounds like hemoglobin, vanadium, and copper.