there are 4 different blood types. they cant always interact with each other. o is a universal donor but some types cant mix with other types. much like chemicals.therefore in a way our blood is like a chemical and like any other chemical needs different instructions.i came across the blood type lifestyle series by dr d'adamo. this is not a diet but a lifestyle. he explains how the 4 types came into existence and different foods that react and dont react well. everyone i have told about this that followed there "diet" to a t has lost weight and improved overall health. i have gone from 385 lbs to 240 following these guide lines for my blood type. This is just food for thought if anyone is interested.