Life is made up of three main components: proteins, sugars, and fats.
Proteins are long chains of carbon, nitrogen and oxygen that form a cell's machinery. Sugars are rings of carbon and oxygen that are used for energy or linked together to form structural support. Fats are long chains of carbon with charged groups at the end and are used for energy or to form membranes that organize living cells. Lectins are a special type of protein that evolved to bind very strongly to specific sugars.
Because of their role in binding sugars, lectins can be important in any biological situation in which sugars are involved. Sugars, or carbohydrates, get their name from the ratio of their chemical composition; for every carbon atom in a sugar, there are two hydrogens and one oxygen (i.e. the equivalent of a water molecule). Although their chemical formula remains the same, carbohydrates exhibit incredible diversity in their structure and function. Simple sugars are made of one carbon ring but have their attached oxygen atoms directed in different orientations, leading to molecules such as glucose, galactose, or allose. All three of these simple sugars have the exact same chemical formula but function very differently; glucose is the most common energy currency for living cells whereas allose is a rare sugar that is found as part of a chemical in an African shrub. In addition to the complexity of simple sugars, carbohydrates can be linked together into disaccharides such as maltose, which is made of two glucose sugars and found in malt beverages. Linking more sugars together leads to the creation of compound sugars, which can be made of hundreds to thousands of simple sugars linked together.
One of the main uses of compound sugars is to maintain the integrity of the cell or organism. Plants use cellulose in order to provide structural support, insects use chitin to form hard and protective exoskeletons, bacteria use sugar capsules outside their membranes to protect them from environmental stress, and humans use mucus to protect the gastrointestinal tract. Since different kingdoms of life use different sugars to accomplish similar goals, lectins provide a way for the human immune system to monitor for infection. Mannose binding lectin binds to sugars found in pathogens and activates the immune system, providing a fast response to infection. At least two studies have shown that mutations in a person's mannose binding lectin can increase his or her susceptibility to some fatal diseases. Unfortunately, lectins can also cause harm to humans by helping infectious diseases and toxins target human cells. The toxin ricin, found in castor oil plant seeds, is a protein made of two chains: the A chain prevents cells from making essential proteins while the B chain is a lectin that binds to sugars on the surface of human cells. Without the B chain lectin to guide the A chain to human cells, the compound loses nearly all of its toxicity, illustrating the important role that this lectin plays.
Foods High in Lectins
Because of their ubiquitous presence in nature, lectins can be found in many of the foods we eat, although some have much higher levels than others. Beans and grains contain high levels of lectins, so much so that if they are eaten raw they can cause severe gastrointestinal discomfort. Tomatoes and other nightshade plants also contain elevated levels of lectins, although not nearly as high as beans, and can be safely eaten raw. Because there are many different types of lectins with varying biological functions, it is difficult to generalize whether lectins in foods are beneficial or harmful. With more research on lectins, it may be possible to harness their power to improve the foods we eat, enhance the immune system, and treat disease.