Lectins as defence molecules in vertebrates and invertebrates

The use of lectins for detecting structural variations in the glycocalyx of interacting cells is common to all forms of life, and forms the basis for their function in fertilisation, development, leucocyte migration and self/non-self distinction. Six lectin families are recognised; legume lectins, cereal lectins, P-, S- and C-type lectins, and pentraxins. Of the latter four occurring in animals, only pentraxins and C-type lectins are implicated in defence. Pentraxins have been described from the horseshoe crab and tunicates, as well as trout and Xenopus. C-type lectins are known from at least four protostome and three deuterostome invertebrates. A role in defence is strongly implicated for at least some of these. The C-type molecules known from lower vertebrates have generally lost their lectin function and exist as antifreeze proteins in fish and as a component of snake venom. However, a factor with structural and functional resemblance to mammalian mannan-binding protein (MBP) has been described from the Atlantic salmon,Salmo salar. For a more complete understanding on the evolution of mammalian C-type lectins, attention should be paid not only to humoral but also membrane bound lectins and extracellular matrix components in invertebrates and lower vertebrates.