A class of proteins of nonimmune origin that bind carbohydrates reversibly and noncovalently without inducing any change in the carbohydrate. Since their discovery in 1888, lectins have been labeled agglutinins, antibodylike molecules, hemagglutinins, heteroagglutinins, heterophile agglutinins, natural or normal antibodies, protectins, phytohemagglutinins, and receptor-specific proteins, among other names. The main feature of lectins is their ability to recognize and bind specific carbohydrate structures becoming the translators of the sugar code (the coding of biological information by sugar structures). Lectins bind a variety of cells having cell-surface glycoproteins (carbohydrate-bound proteins) or glycolipids (carbohydrate-bound lipids). The presence of two or more binding sites for each lectin molecule allows the agglutination (clumping) of many cell types, and the agglutination reaction has been used extensively to detect the presence of lectins in extracts from different organisms. However, multivalency (having several sites of attachment) may not be an absolute requirement, even though it is still an important factor for most lectins. Lectins are ubiquitous in nature and may have very different roles according to the organism, tissue, or developmental stage. Their potential ligands, simple or complex carbohydrates, are present in all living cells and in biological fluids, which suggests that protein–carbohydrate interactions constitute basic phenomena common to all organisms.