Since the 1880’s, it has been known that extracts from certain plants could agglutinate red blood cells. In the 1940’s, agglutinins were discovered which could “select” types of cells based on their blood group activities. Although “lectin” was originally coined to define agglutinins which could discriminate among types of red blood cells, today the term is used more generally and includes sugar-binding proteins from many sources regardless of their ability to agglutinate cells. Lectins have been found in plants, viruses, microorganisms and animals, but despite their ubiquity, their function in nature is unclear. Although lectins share the common property of binding to defined sugar structures, their roles in various organisms are not likely to be the same.   Most lectins studied to date are multimeric, consisting of non-covalently associated subunits. A lectin may contain two or more of the same subunit, such as Con A, or different subunits, such as Phaseolus vulgaris agglutinin. It is this multimeric structure which gives lectins their ability to agglutinate cells or form precipitates with glycoconjugates in a manner similar to antigen-antibody interactions. Although most lectins can agglutinate some cell type, cellular agglutination is not a prerequisite. Some lectins can bind to cells and not cause agglutination, such as succinylated Con A, or the lectin may not bind to cells at all. The latter property may be a consequence of the structure of the lectin or the absence of a suitable receptor oligosaccharide on the cell surface. Since agglutination of cells is the assay most generally employed to measure lectins, many non-agglutinating lectins may exist in nature but have not been easily detected.   In spite of our ignorance about the function of lectins in nature, this unique group of proteins has provided researchers with powerful tools to explore a myriad of biological structures and processes. Because of the specificity that each lectin has toward a particular carbohydrate structure, even oligosaccharides with identical sugar compositions can be distinguished or separated. Some lectins will bind only to structures with mannose or glucose residues, while others may recognize only galactose residues. Some lectins require that the particular sugar be in a terminal non-reducing position in the oligosaccharide, while others can bind to sugars within the oligosaccharide chain. Some lectins do not discriminate between a and b anomers, while others require not only the correct anomeric structure but a specific sequence of sugars for binding. The affinity between a lectin and its receptor may vary a great deal due to small changes in the carbohydrate structure of the receptor. All of these properties which are peculiar to lectins enable the researcher to discriminate between structures, to isolate one glycoconjugate, cell or virus from a mixture or to study one process among several. Since virtually all biological membranes and cell walls contain glycoconjugates, all living organisms can be studied with lectins.   Another property of some lectins is an ability to induce mitosis in cells which are normally not dividing. This property has been exploited extensively in an attempt to understand the process of lymphocyte blastogenesis and the biochemical and structural alterations associated with mitogenesis. It is not clear why some lectins are mitogenic since the structures to which mitogenic lectins bind are not necessarily the same, and not all lectins with similar binding specificities are mitogenic. It is likely that binding to the cell surface alone is not sufficient to cause mitosis but that other interactions on the cell surface are equally important.   Additional Information:   Purification of Lectins   Description of Agarose Bound Lectins   Description of Fluorescent Lectins   Description of Peroxidase Conjugated Lectins   Specificity Guide for Lectins   General Lectin References   Lectin Perfusion References (Tomato Lectin)   Protocols:   Peroxidase localization of WGA in neural tissue:   L1020.pdf (PDF)   Enrichment of NK cells by panning on ECL plates:   L1140.pdf (PDF)   Specificity of Jacalin binding:   L1150.pdf (PDF)