The ABO grouping system is also classified into four types based on the presence or absence of antigens on the red blood cells(RBC) surface and plasma antibodies.
Outside of the ABO system, there are hundreds of antigens that may be attached to red blood cells.
Some of the antigens are so rare that they are found in very few people.
One of the world's rarest blood types is Rh- null. It is called the "golden blood".
This blood type is distinct from Rh-negative since it has none of the Rh antigens at all.
There are less than 50 people in the world population who have this blood type.