SkepticRD has over 15 years experience in the field of nutrition; long enough to not only see a lot of quackery but also to see the same quackery come around again and again. One of the books that keeps coming back to haunt me and other Fighters of Food Woo is the book "Eat Right For Your Type" by Peter D'Adamo, ND, first published in 1996. And no, I am not even going to dignify this book with a link.
As far as why this book is wrong on so many levels, this has already been covered by many authors who are devoted to skepticism and anti-quackery. Probably the best summary that SkepticRD has encountered is this article done done for The Skeptic's Dictionary: Link.
In summary of that summary:
1) If those of us who had been eating the "wrong" foods really are suffering from the multi-organ failure from lectins and agglutinations it would show up on the same type of imagining used to diagnose atherosclerosis and also on autopsies. It doesn't.
2) D'Adamo claims that "O" is the oldest blood type, however, studies in humans and other primates indicate that alleles coding for type "A" are the most ancient. Given that hunter-gatherer cultures are much older than agrarian types, and the hunter-gatherer diet is not suitable for type A's in D'Adamo's head, then this is not a scientific fact is it.
3) Pathologist Karl Landsteiner identified the different blood types in the earlier part of the twentieth century by classifying blood on the presence or lack thereof of different antigens. Since then, over 276 red blood cell antigens have been identified. Notice there are only 4 different types of diets in this book, not 276 different plans.
4) The author goes beyond diet into a type of "blood type astrology" attributing different personality characteristics to various blood types. This includes the use of the terms "master race" and stating that Type A's tend to become paranoid like Hitler; that should be enough to raise red flags in the mind of any one with a drop of skeptical blood in their body (pun intended).
SkepticRD is even more interested in why people keep falling for this kind of quackery. Some of my thoughts include:
1) People who struggle with their weight want to find something to do that will cease their struggles. The idea of finally finding something that doesn't involve counting carbohydrate grams, etc sounds very appealing. **Sidebar** There is a predominant myth that obese/overweight people are lazy. If you would see some of the things people go through to lose weight, you would likely not believe that. When a person says "I've tried everything" I often find them to be right.
2) One person may do better with one diet plan than another person, and we as humans want to know why that is. Throwing around the terms "antigen" and "evolution" can sound like quite a scientific solution to those who do not know much about human physiology or evolutionary biology.
3) People don't question authority or tiles that sound like they should have authority. Typically when I see ND ("Doctor" of Naturopathy) my Baloney Detector siren starts ringing as I know pseudoscience abounds; see Quackwatcher Stephen Barrett's article here. Also, when I asked a co-worker why she was reading that book (and this was a health care professional) she stated "A doctor at my church recommended it."
4) There are just enough healthy recommendations in it that it sounds like it could actually be healthy--none of the groups are supposed to consume added sugars, white flour, fast food, etc. Most of could stand to do that, but it doesn't mean that if a type O eats dairy that his/her organs will suddenly fail.
5) People falling victim to the fallacy of "it's natural so it must be better." Time for a cliche: "You know what else is natural? Arsenic!"
6) People falling victim to the "ancient wisdom" fallacy. There was once a time when people sacrificed children to stop natural disasters, but we wouldn't think of doing something similar today. People also used to live without refrigeration (many cultures still do!) but we in western society find that a necessity for keeping our food supply healthy. Ancient does not necessarily mean better.
What can we do to stop the revolving door of quackery? Try to understand people's motivations, and try to research the wonderful world of science behind it. And if you're wanting to find out your blood type, go do something useful and life giving in the mean time, like donate blood!