Lectins occur in plants animals bacteria and viruses. Initially described for their carbohydrate-binding activity they are now recognised as a more diverse group of proteins some of which are involved in protein-protein protein-lipid or protein-nucleic acid interactions . There are at least twelve structural families of lectins:
- C-type lectins which are Ca+-dependent.
- S-type (galectins) a widespread family of glycan-binding proteins .
- I-type which have an immunoglobulin-like fold and can recognise sialic acids other sugars and glycosaminoglycans .
- P-type which bind phosphomannosyl receptors .
- Pentraxins .
- (Trout) egg lectins.
- Calreticulin and calnexin which act as molecular chaperones of the endoplasmic reticulum .
- ERGIC-53 and VIP-36 .
- Discoidins .
- Eel aggutinins (fucolectins) .
- Annexin lectins .
- Fibrinogen-type lectins which includes ficolins tachylectins 5A and 5B and Limax flavus (Spotted garden slug) agglutinin (these proteins have clear distinctions from one another but they share a homologous fibrinogen-like domain used for carbohydrate binding).
- Also unclassified orphan lectins including amphoterin Cel-II complement factor H thrombospondin sailic acid-binding lectins adherence lectin and cytokins (such as tumour necrosis factor and several interleukins).
C-type lectins can be further divided into seven subgroups based on additional non-lectin domains and gene structure: (I) hyalectans (II) asialoglycoprotein receptors (III) collectins (IV) selectins (V) NK group transmembrane receptors (VI) macrophage mannose receptors and (VII) simple (single domain) lectins
.Therefore lectins are a diverse group of proteins both in terms of structure and activity. Carbohydrate binding ability may have evolved independently and sporadically in numerous unrelated families where each evolved a structure that was conserved to fulfil some other activity and function. In general animal lectins act as recognition molecules within the immune system their functions involving defence against pathogens cell trafficking immune regulation and the prevention of autoimmunity