The International Society of Blood Transfusion recognizes 35 different blood type groups based on various antigens (or proteins) on the surface of red blood cells, yielding millions of individual blood types.
The two most important blood type systems are ABO and the Rhesus D system that gives the positive/negative classification.
The rarest type of all is called Rhnull blood which lacks any of the more than 50 Rh-related antigens. This blood is extremely valuable because it is truly universal and can be donated to anyone, including patients with other rare blood types.
How many people have Rhnull blood? Just over 40 people on the entire planet Earth have been found to have it. At last count, there were 9 active donors.
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