Lectins are carbohydrate binding proteins present in most of the plants and in some animals. The word Lectins is derived from Latin- Legree = to pick or select. Lectins do not cause any antigenic stimulation within the immune system but they have the basic capacity to bind analogously to an antibody.
Several approaches emphasize that the cell glycoconjugates have fundamental roles in a variety of cell functions, including development, growth regulation, and cellular locomotion . These lectins selectively bind carbohydrate moieties of the glycoprotein that decorate the surface of most of the animal cells. The specific capacity of lectins to bind with the cell surface mainly depends on the monosaccharides or simple oligosaccharides, which when present, inhibit the lectins associated reaction. Structurally, these lectins have a diverse class of proteins, which have the ability to bind carbohydrates with considerable specificity ; therefore, they are used as histochemical reagents to investigate the distribution of glycoconjugates and carbohydrates in various tissues. The lectins have also been defined as "proteins of non-immunoglobin nature capable of specific recognition and reversible binding to carbohydrate moieties of complex carbohydrate without altering the covalent structure of any of the recognized glycosyl ligand. These proteins of plants, vertebrate and invertebrate origin have specific affinities for particular terminal sugar residues or specific sugar sequences . For this unique property, lectins have been introduced as histochemical reagents to study the distribution of glycoconjugates in various normal and malignant cells and tissues. It is extremely interesting and significant that most plant and animal lectins have been classified into a rather limited number of carbohydrate-binding groups. These include the mannose / glucose-binding lectins, the galactose-binding lectins, N-acetylgalactosamine-binding lectins, the N- acetyl glucosamine binding lectins ,the L-fucose-binding lectins, sialic-acid-binding lectins and lectins with complex carbohydrate binding sites .