Final answer:
The fourth important blood group antigen, in addition to A, B, and O, is the Rh antigen.
Step-by-step explanation:
In addition to the three main blood group antigens A, B, and O, there is another important antigen known as the Rh antigen. The Rh antigen is named after the Rhesus monkey, which is where the antigen was first discovered. The presence or absence of the Rh antigen on red blood cells determines whether a person is Rh-positive or Rh-negative.
The Rh antigen is important in blood transfusions and during pregnancy. For example, if a person is Rh-negative and receives Rh-positive blood, the person's immune system may produce antibodies against the Rh antigen, leading to a transfusion reaction. In pregnancy, if a Rh-negative mother carries a Rh-positive baby, the mother's immune system may produce antibodies that can harm the baby.
In addition to the ABO and Rh antigens, there are many other blood group systems that have been identified, but they are generally considered less important in routine blood typing and transfusion procedures.