Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Lipids and Membranes
Lectins are specific proteins capable of binding to specific cells and inducing their agglutination.

Many years ago, it was established that Certain plant Proteins, often referred to as phytohemagglutinins, are capable of binding to erythrocytes and causing their agglutination. It was later discovered that phytohemagglutinins can attach to The surface of many other animal Cells. Although proteins of this type are found mainly in plants, particularly in members of the legume family, they also occur in invertebrate Tissues. Among the first discovered phytohemagglutinins were concanavalin A from the jack bean (Canavalia ensiformis) and ricin from the castor oil plant (Ricinus communis). These and many other PLANT AND ANIMAL proteins capable of binding to specific carbohydrate groups on The Cell surface became known as Lectins (from the Latin legere, meaning to pick or choose). For example, concanavalin A binds to D-glucose and D-mannose, while soybean lectin binds to D-galactose and N-acetyl-D-galactosamine. Today, over 1,000 lectins have been identified. Of particular interest and importance is the fact that certain lectins cause the selective agglutination of malignant tumor cells. This points to structural differences between the surfaces of tumor and normal cells: apparently, specific carbohydrate residues On the surface of tumor cells are more accessible to the lectins that bind to them.

The Specificity of lectins, as well as their significant practical importance, is manifested in their ability to distinguish among three erythrocyte types—A, B, and O—based on The Structure of the oligosaccharide groups of Glycophorin. Limabean lectin agglutinates only type A erythrocytes, lotus lectin agglutinates type B erythrocytes, and a third lectin, also of plant origin, agglutinates only type O erythrocytes. Lima-bean lectin binds to Glycoproteins containing N-acetyl-D-galactosamine as their major sugar component, whereas lectins reacting with type O erythrocytes bind to surface glycoproteins containing the sugar fucose. Specific Glycoproteins are present on the surface not only of erythrocytes, but also of other cells in animal tissues. It is precisely because of these glycoproteins that, when Skin or an organ (such as a Kidney or Heart) is transplanted from one person to another, donor-recipient tissue compatibility is required for the successful engraftment of the transplanted tissue.

In plants and invertebrates, lectins likely function as protective proteins, safeguarding these organisms—which lack an immune system and, consequently, Antibodies—from invasion by parasitic microorganisms. It is believed that lectins are located on the surface of plant cells.