I remember when I read about this years ago the author claiming that eating the wrong diet for your blood type causes agglutination, which...no. Agglutination is huge, it's what happens when a person is given the wrong type of blood during a transfusion. It's effects are not subtle or gradual, it's not something that would ever be missed by a doctor. I might (okay not really (what, no Type As have Celiac?) but let's pretend) be willing to believe that a person's blood type should determine their diet, but I'm going to need an actual reason for it. Throwing out big words to impress the average lay person isn't going to cut it.

I mean, if one notices a positive difference by avoiding some foods then (I mean real positive differences, not like what Gwyneth Paltrow considers real), yeah, go for it. But basing it on blood type doesn't make any sense.
Eres o te haces?