AB- (AB negative) is one of the eight main blood types in the ABO blood group system. It is characterized by the absence of the A and B antigens on the surface of red blood cells, as well as the absence of the Rh factor (making it negative). This blood type is relatively rare, found in about 1% of the population. Individuals with AB- blood can receive blood from all negative types, including A-, B-, O-, and other AB types. However, they can only donate blood to other AB blood types, making them universal plasma donors due to their unique antibody composition.