Answer:
It will be simpler if you would know what exactly is blood group. These blood group are defined according to the antigen present or absent for ex A blood group will have A antigen but B has B ,ÀB has both antigen but O does not have any . Whenever a person of A blood group recieves blood of A blood group the body can't tell the difference. But if he recieves blood from a different blood group say B it is recognised as foreign by body and a reaction occurs (the reaction is similar to that against a foreign antigen ex bacterial antigen).Same stands for B blood group
Since AB blood group has both the antigens it does not show any reaction. O has none it shows reaction to both of them.
This is based on blood grouping according to ABO system. There are about 1000 blood group system present in humans.
Hope it helps!!!