In an emergency situation where a patient's blood group is unknown, it is critical to transfuse blood that is compatible with all blood types to avoid a potentially fatal immune reaction. Red blood cells of type O negative (O-) are considered the universal donor because they lack the A, B, and Rh antigens. This means they can be safely transfused to individuals of any ABO blood group (A, B, AB, or O) and any Rh type (Rh+ or Rh-) without causing agglutination or haemolysis. Type O+ blood can be given to Rh+ individuals of any ABO type, but not to Rh- individuals. AB+ blood is the universal recipient for red blood cells but is not a universal donor. AB- blood can receive from O-, A-, B-, and AB- but is only a donor for AB+ and AB-.