Particular proteins or other complex molecules generated by one cell type that another cell type has receptors for and can ‘recognise’ (i.e., receptor uptake of these molecules causes some change in the cell’s physiology, usually by genetic instruction). They mediate cell-cell and cell-extracellular matrix interactions. It appears that are a very large number of genes in the vertebrate genome that encodes for these molecules, which might be an outcome of increasingly more complex cellular interactions being selected for in the evolution of ‘higher’ metazoans.
See Cell, Cell adhesion molecules (CAMs), Metazoan, Proteins