ABO blood grouping system is based on presence or absence of three types of antigen on red blood cells. A and B blood group have A and B antigens respectively. AB blood group has both A and B antigens. O blood group does not have any antigen.
A body with particular blood type will consider other blood group foreign ( because it has different antigen ) and produce antibodies against it. A person with blood type A can not be given B type blood because the antibodies in his body will destroy RBCs with B antigen which may even prove fatal.
AB blood type person can receive blood from both A and B blood groups because he has antigens for both of them so it will produce antibodies for neither of them. He can also receive O type blood because O blood does not have any antigen so it can be given to any person without incompatibility reaction.
However, AB individual can only donate blood to AB individual because his blood cells have both A and B antigens. So, he can only donate blood to an individual who does not produce antibodies for both A and B blood groups and that would only be another AB blood type individual.
