The rarest blood type is AB negative (AB-). Blood types are determined by the presence or absence of antigens on red blood cells, and the AB- blood type is found in less than 1% of the global population. Blood type distributions can vary among different ethnic and cultural groups, but AB- remains one of the most uncommon types overall. The rarity of AB- can complicate blood transfusions, especially in emergencies, as the demand for specific blood types can exceed the supply. Understanding blood types is critical for medical practices and transfusion safety.