A substance that is derived from a plant and causes red blood cells to clump together.
Specifically, it is the lectin (sugar-binding protein) obtained from red kidney beans.
Etymology: phyton (plant) + haima (blood) + agglutinare (to glue); the first two words are Greek but agglutinare is Latin
When properly prepared, beans are healthy, tasty, and an inexpensive source of protein. But they should be fully cooked.
Many kinds of beans contain phytohaemagglutinin; red kidney beans contain the highest levels by far.
Consuming raw or undercooked kidney beans can lead to symptoms in as little as 1 to 3 hours. Symptoms include nausea, vomiting, and diarrhea, which can be severe and debilitating. Recovery is usually fast (3-4 hours after onset of symptoms) and spontaneous. As few as 4 or 5 beans can trigger symptoms.
To remove lectins, livestrong.com suggests soaking beans overnight, then boiling for a minimum of 10 minutes. The beans need to be non-crunchy and sufficiently warmed through.
My own process for cooking dry beans: sort (check for and remove rocks, non-beans, and shriveled beans) and rinse, then boil them in water for 2-3 minutes, let beans sit for an hour or so, then rinse and add new water and bring back to a boil. When they are at the desired softness, add salt (salt adds flavor, but generally stops the beans from getting any softer). Tiny beans like lentils don't need pre-boiling. I always cook beans fully before adding them to a crockpot - salt the beans before adding and they'll keep their shape.
Other foods also contain lectins, such as cereal grains, seeds, nuts, and some fruits. Soybeans contain soybean lectin. With some lectins, the reactivity varies depending on blood type.