Explain ABO blood grouping.

In 1901, Karl Landsteiner classified blood into four blood groups. This system is called ABO system. It is based on the presence or absence of a surface antigen, on the surface of red blood cells. According to this, there are four blood groups, A , B, AB and O. The A blood group has A antigen, B blood group has B antigen. AB has both whereas O blood group has no antigen on RBC surface.