Collectin

Collectins are soluble pattern recognition receptors (PRRs) belonging to the superfamily of collagen containing C-type lectins.

Eight collectins have been identified including mannan-binding lectin (MBL), surfactant protein A (SP-A), surfactant protein D (SP-D), collectin liver 1 (CL-L1), collectin placenta 1 (CL-P1), conglutinin, collectin of 43 kDa (CL-43) and collectin of 46 kDa (CL-46).

These molecules have been implicated as major modulators of the innate immune system where they have a key role in the first line of defense against invading microorganisms by binding to them so macrophages know to dispose of them.

Functionally, collectins are usually trimers with the number of trimeric units differing among the collectin family.

Structural domains
The collectin monomers comprise four structural domains;
 * a cysteine-rich domain at the N-terminus.
 * a collagen domain.
 * a coiled-coil neck domain.
 * a C-type lectin domain that is also called a carbohydrate recognition domain (CRD). Collectins selectively bind to specific complex carbohydrates of microbes using their CRDs.

Responses
The binding of collectins to microbes causes several immune related responses.


 * They cause the formation of aggregates of the microorganisms, opsonize the microorganisms to increase their phagocytosis and some upregulate of the activity of other PRRs such as the mannose receptor.


 * Collectins can also induce the production of pro-inflammatory molecules like cytokines, and reactive oxygen species in phagocytes by interacting with other cell surface receptors or by scavenging of bacterial molecules like lipopolysaccharide (LPS).


 * Some collectins (e.g. MBL) activate the lectin pathway of the complement system to increase membrane permeability of microorganisms and cause the destruction of that microbe.