Lectins are proteins common in seeds. They bind to sugars attached in chains to proteins, i.e. glycoproteins, and are displayed on the surfaces of cells that line the gut. Lectins could inhibit digestion of raw beans, but cooking makes them digestible.
Fear of lectins is puzzling. Lectins are proteins that have binding sites on their surfaces for specific single or small sequences of sugars. They are present in seeds to protect the seeds from herbivores.
A seed is mostly food (starch, protein, fat) for the plant embryo that will grow from it. This is also true of a chicken egg and just like the egg, the seed contains defensive proteins to inhibit the growth of bacteria, fungi and egg/seed eaters.
The egg has enzymes to degrade bacterial walls and proteins that bind iron, vitamins, etc. needed by bacteria and humans. Eating many raw eggs can lead to vitamin deficiencies. Boiling the eggs, unravels the defensive proteins and makes them digestible and nutritious.
Seeds block being digested by containing proteins that foul the digestion system of would be devourers. For example, soybeans have trypsin inhibitor that binds to our digestive enzyme and makes eating raw soybeans nonproductive and uncomfortable. Boiling soybean meal to produce a curd, i.e. tofu, agglutinates the denatured soy proteins, including the lectins and washes away the soy trypsin inhibitor. Tofu is free of digestion inhibitors and lectin activity.
It is not an accident that lectins bind to human red blood cells. The sugars displayed on the surface of red blood cells are the blood group antigens. Different sugars on the end of the sugar chains decorating RBCs characterize the A, B and O antigens. These same sugars are present on the surfaces of various bacteria. Immune systems don’t produce antibodies to self antigens, so a person with type A blood produces antibodies only to B antigen sugars it encounters on bacteria. A person who is type AB doesn’t produce antibodies to A or B sugar antigens. There aren’t antibodies to O, because that sugar structure is the basis upon which both A and B are made, and some of the
structure is present on all RBCs. Lectins are specific for A or B or other common bacterial sugar antigens.
I did some modeling to show a lectin with lactose (red and gray) bound to sticky tryptophans (yellow) in two places on the surface. In one case a lysine (blue) is draped on the other side. That shows that sugars bind both to aromatic amino acids and to the hydrophobic arms of basic amino acids.
Some people think that humans and other mammals must be protected from lectins and that this protection is shown in human and cow’s milk in the form of antibodies against lectins. This seems to be a misunderstanding. For example, human antibodies secreted in breast milk are secretory IgAs. These antibodies are glycoproteins, i.e. they are proteins with attached sugar chains. Some lectins will bind to these antibodies, because of the attached sugars. These are not lectin-specific antibodies, but rather glyco-specific lectins. The lectins are binding to the glycoprotein antibodies, not the other way around.
It is possible for people to be allergic to lectins, but this is unlikely. For example, peanut allergies involve proteins other than the peanut lectins.
There are some dangerous lectins. For example, ricin is a very nasty, but effective, toxin produced by the castor plant. Ricin is a lectin, in that it binds very specifically to sugars found on the surface of gut cells of insects and humans. After the ricin binds to surface proteins, it is brought into the cells where it chops up the protein synthesizing machinery. That is a dangerous lectin. It takes very little ricin to kill each cell and only a tiny amount to kill a human. Ricin is a terrorist toxin. Yet oil extracted from castor beans contains so little contaminating ricin that it is safe to eat. [Castor oil is wonderful to apply to aching feet overnight for painfree, luxuriously soft feet in the morning.]
The bottom line is that seed lectins add to the nutrition of cooked beans and grains that have been the foundations for several thriving civilizations. The longest living members of the bean and grain cultures are typically older and more fit than comparable individuals with a modern, inflammatory diet based on omega-6 oils.