There's a big following for this diet with absolutely no scientific proof. Here's an article written by a real doctor that prescribes life-saving diets with the best explanation:
by Michael Klaper, M.D.
The "blood type diet" theory has gained widespread attention from the public since the release of "Eat Right For Your Type" by Peter J. D'Adamo, N. D. (G. P. Putnam's Sons, New York, 1996). The book's basic premise - that Type O's are the dominant, hunter-caveman type that require meat in the diet, Type A's are docile vegetarians, and Type B's are dairy-eating omnivores - has become a dietary manifesto for many people. However, the "blood type diet" theory, and the book that promotes it, presents many problems that prevent me from seriously basing any of my dietary choices upon them.
Read the full article here: http://www.earthsave.org/health/bloodtyp.htm
Dr. Michael Klaper is a practicing physician with forty years of experience in general practice and acute care medicine.