I recommend that you read the first few chapters of "Eat Right 4 Your Type" that explain the theory behind it. I thought it was a lot of bunk before I read it but since we had the book in my house I read it anyway. After reading it, it made a lot of sense. Yes, the idea is what Katherine K. said, your blood recognizes some substances as foreign and attacks them. This reaction could be more extreme in some individuals than others, but the idea still holds true. The most telling factor was that most of the foods I'm not supposed to eat I already avoid, and then the few that I had been chomping down on I had been noticing they would make me feel sick and my skin break out so I just went cold turkey from them and now I feel great, also my allergies have pretty much 100% gone away. It wouldn't hurt to just read the book and give it a try, reading the book is my best recommendation.
By the way it is not a lose-weight diet, it is just for health. Also somewhat anti-inflammatory, should help a variety of different problems.