A lectin is a type of protein molecule found in many types of plants, grains and animals. They are a type of 'molecular velcro,' capable of attaching to specific sugars found on the surfaces of many cells, including the cells of the bloodstream  and digestive tract. This attachment can often result in the affected cells sticking together (agglutination) which can cause tissue destruction, inflammation, and wreak hormonal havoc. 
You may have heard about lectins recently but Dr. Peter D'Adamo has been talking and writing about them for decades. Lots of nutrition books now talking about lectins. 'Eat Right' was the first, over twenty years ago. Nice to see they're catching up, but most conveniently don't mention that lectin specificity is often to a particular ABO blood type antigen. Giving dietary advice about lectins but not including blood type specificity is like giving just your city and state to someone who is asking for your mailing address. The letter is very unlikely to ever arrive!