Antigens are associated with: bacteria, pollen, foreign blood cells, toxins, viruses, and the cells of transplanted organs.Red Blood Cell (RBC) antigens are attached to components in the membranes of the cells. (ABO blood group are sugars; Rh group are polysaccharides.)
Blood grouping is determined by the antigens (aka surface markers) found on RBCs. If a person has, for example, type A blood group, their immune system makes antibodies only for type B surface antigens. These antibodies do not recognise (key doesn't fit lock) type A antigens, so they ignore them. The type A person can receive blood from donors of types A or O, and can donate to recipients with types A or AB. The parallel situation occurs in people with type B blood.
People with type AB blood do not make antibodies for either type A or type B antigens, which means that they may receive blood from donors of any type - they are universal recipients.