The underlying theory of blood type diets is that people with different blood types digest lectins differently, and that if people eat food that is not compatible with their blood type, they will experience many health problems. On the other hand, if a person eats food that is compatible, they will be healthier.

That theory is, in turn, based on an assumption that each blood type represents a different evolutionary heritage. "Based on the ‘Blood-Type’ diet theory, group O is considered the ancestral blood group in humans so their optimal diet should resemble the high animal protein diets typical of the hunter-gatherer era. In contrast, those with group A should thrive on a vegetarian diet as this blood group was believed to have evolved when humans settled down into agrarian societies. Following the same rationale, individuals with blood group B are considered to benefit from consumption of dairy products because this blood group was believed to originate in nomadic tribes. Finally, individuals with an AB blood group are believed to benefit from a diet that is intermediate to those proposed for group A and group B.

As of 2013 there is no scientific evidence to support the blood type diet theory and no clinical evidence that it improves health. Peter J. D'Adamo is the most prominent proponent of blood type diets and he described ongoing clinical trials in books he published in 1996 and 2004; however no clinical trial results have been published.

The claim made in blood type diets, that people with different blood types react differently to lectins, is unsubstantiated by established biochemical research, which has found no differences in the reactions of lectins with human ABO types. Research shows that lectins specific to a particular ABO type are generally not found in foods (with several rare exceptions, e.g., the Lima bean) and that lectins with ABO specificity are more frequently found in non-food plants or animals.

Luiz C. de Mattos and Haroldo W. Moreira point out that assertions made by proponents of blood type diets that the O blood type was the first human blood type requires that the O gene have evolved before the A and B genes in the ABO locus; phylogenetic networks of human and non-human ABO alleles show that the A gene was the first to evolve. They argue that it would be extraordinary, from the perspective of evolution, for normal genes (those for types A and B) to have evolved from abnormal genes (for type O).

Yamamoto et al. further note: "Although the O blood type is common in all populations around the world, there is no evidence that the O gene represents the ancestral gene at the ABO locus. Nor is it reasonable to suppose that a defective gene would arise spontaneously and then evolve into normal genes.

And

I've heard of this theory b4. Turns out to not be backed up by any any scientific evidence .
My take, eat on a balanced diet and keep away from things ur body seems to be rejceting or allergic to