A- (Blood) is one of the four main blood types in the ABO blood group system, which also includes A+, B+, AB+, and O+. Blood type A- means that the person has A antigens on the surface of their red blood cells but lacks the Rh factor, which is a specific protein. This blood type is relatively rare, occurring in about 6% of the population. Individuals with A- blood can receive blood from A-, A+, O-, and O+ donors. It is important for blood transfusions to match the recipient's blood type to prevent adverse reactions. Blood type compatibility is crucial in medical situations, such as surgeries or emergencies, where blood transfusions may be necessary.