According to the ABO blood group system there are four different kinds of blood groups: A, B, AB or O. The differences in human blood are due to the presence or absence of certain protein molecules called antigens and antibodies. The antigens are located on the surface of the red blood cells and the antibodies are in the blood plasma.
A person with blood group A has A antigens on the red blood cell surface and B antibodies in the blood plasma. Anti-A antibodies will not be produced by them because they would cause the destruction of their own blood. However, if B type blood is injected into their systems, anti-B antibodies in their plasma will recognize it as alien and burst or agglutinate the introduced red cells in order to cleanse the blood of alien protein.
Blood group B --> B antigens on red cell surface and anti-A antibodies in blood plasma
Blood group AB --> Both A and B antigens on red blood cell surface and no antibodies at all in blood plasma
Blood group O --> Neither A or B antigens on the red blood cell surface and both anti-A and anti-B antibodies in blood plasma.