Collectin
Collectins (collagen-containing C-type lectins) are a part of the innate immune system. They form a family of collagenous Ca2+-dependent defense lectins, which are found in animals. Collectins are soluble pattern recognition receptors (PRRs). Their function is to bind to oligosaccharide structure or lipids that are on the surface of microorganisms. Like other PRRs they bind pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) of oligosaccharide origin. Binding of collectins to microorganisms may trigger elimination of microorganisms by aggregation, complement activation, opsonization, activation of phagocytosis, or inhibition of microbial growth. Other functions of collectins are modulation of inflammatory, allergic responses, adaptive immune system and clearance of apoptotic cells.
Functionally collectins are trimers. Monomeric subunit consists of four parts:
Recognition of specific parts of microorganism is mediated by CRD in presence of calcium. Affinity of interaction between microbes and collectins depends on the degree of collectin oligomerization and also on the density of ligands on the surface of the microbe.
Nine types of collectins have been defined:
CL-43, CL-46 and conglutinin are found in bovine.
Collectins can bind on the surface of the microorganism and between carbohydrate ligands can be made a bond. Due to those properties, the interaction can result in aggregation.
Collectins can act as opsonins. There is a specific interaction between collectins and receptors on phagocytic cells which can lead to increased clearance of microorganisms. MBL can bind to microorganisms and this interaction can lead to opsonization through complement activation, or it can opsonize the microorganism directly. SP-A and SP-D can also interact with microorganisms and phagocytic cells to enhance phagocytosis of the microorganism.
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