Lectins are proteins that bind to carbohydrates. The occur naturally in most plants. Some foods that contain higher amounts of lectins include beans, peanuts, lentils, tomatoes, potatoes, eggplant, fruits, and wheat and other grains. Lectins serve a protective function for plants as they grow. They don't have any nutritional value when consumed in foods.
In their raw form, lectins can cause negative side effects, including nausea, vomiting, and stomach upset. However, most of the plants that contain high levels of lectins are not consumed in their raw form. Cooking with wet heat (boiling, steaming), or soaking in water for several hours, inactivates eliminates nearly all lectins from food. Lectins are water-soluble and typically found on the outer surface of a food, so exposure to water removes them.