Any of a group of proteins found in plants, animals, fungi, algae, and bacteria that share the property of binding to specific carbohydrate groups. Lectins fall into several categories on the basis of their binding specificities. One large family comprises the calcium-dependent (C-type) lectins, which share a highly conserved carbohydrate recognition domain and require calcium ions to stabilize the binding site. The selectins comprise a subfamily of C-type lectins that function in immune responses as cell adhesion molecules. They are expressed by activated endothelium on the inner surface of blood vessels and bind to passing leucocytes, detaining them for subsequent migration through the vessel wall to an infection site. Also classed as C-type lectins are the collectins (collagen-like lectins); these include mannose-binding lectin, an acute-phase plasma protein that binds to bacterial surfaces, and macrophage mannose receptor, which binds certain sugars on the surface of bacteria and viruses, thereby activating the macrophage. Galectins form another category of lectins, with specificity for galactose. Plant lectins are a heterogeneous group of molecules involved in a wide range of cellular activities. By specifically binding to carbohydrate groups attached to other cell components, including the cell wall, they are crucial in processes such as cell recognition, growth, and division. In legumes lectins take part in the recognition of suitable bacterial partners for the plant in establishing root nodule symbioses. Many plant lectins bind to polysaccharides on the surface of invading organisms and are important in the plant’s defence against fungi and herbivorous animals. Plant lectins can often bind to animal cells, and some are toxic to animals, including humans. A notable example is ricin, obtained from the seeds of the castor oil plant Ricinus communis. Lectins derived from plant seeds, such as concanavalin A, can cause cells to clump together by forming cross links between the oligosaccharide groups on cell surfaces. Lectins are widely used for diagnosis and experimental purposes, e.g. to identify mutant cells in cell cultures, to determine blood groups by triggering agglutination of red blood cells, or in mapping the surface of plasma membranes.