Your blood type reacts to antigens found on other blood types. That's why a type O can't get a transfusion from these other blood types. Type O is not antigenic so it can be donated to anyone. Type AB can take anyone's blood because only the types A,B and O exist. Most food and other ingestible organics such as herbs also have antigens.
Antigens are substances which provoke an immune system into wanting to eliminate it from the body.
They cause an immune reaction. Different foods may react in your body like a foreign blood type and provoke an immune response, depending upon your blood type. This process is called agglutination and it gums up the works, irritates the bowels and clogs up the kidneys. Many diseases are either caused by or aggravated by eating the wrong foods. Likewise, weight gain may be attributable to eating the wrong kinds of foods.