O blood is one of the four main blood types in the ABO blood group system, which also includes A, B, and AB. It is characterized by the absence of A and B antigens on the surface of red blood cells. People with O blood can donate to any other blood type, making it a universal donor type, but they can only receive blood from other O type donors. Individuals with O blood can be further classified into O positive or O negative, depending on the presence of the Rh factor. O negative blood is especially valuable in emergencies, as it can be given to patients of any blood type without risk of an adverse reaction.