Although all blood is made of the same basic elements, not all blood is alike. In fact, there are eight different common blood types, which are determined by the the ABO blood group system and Rh factor.

The ABO Blood Group System

There are four major blood groups determined by the presence or absence of two antigens – A and B – on the surface of red blood cells and two antibody in the plasma:

ABO Blood Type          Antigen A       Antigen B      Antibody anti-A      Antibody Anti-B

               A                      yes                     no                    no                            yes

               B                       no                     yes                   yes                           no

               O                       no                      no                    yes                          yes

              AB                     yes                    yes                    no                            no

Rh Blood Types

Rh blood types were discovered in 1940 by Karl Landsteiner and Alexander Wiener.  This was 40 years after Landsteiner had discovered the ABO blood groups. The Rh system was named after rhesus monkeys, in whose blood this antigen was first found.