Collectins are soluble pattern recognition receptor

Pattern recognition receptor

Pattern recognition receptors are a primitive part of the immune system. They are proteins expressed by cells of the innate immune system to identify pathogen-associated molecular patterns , which are associated with microbial pathogens or cellular stress, as well as damage-associated molecular...

s (PRRs) belonging to the superfamily of collagen

Collagen

Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

 containing C-type lectin

C-type lectin

A C-type lectin is a type of carbohydrate-binding protein domain known as a lectin. The C-type designation is from their requirement for calcium for binding. Proteins that contain C-type lectin domains have a diverse range of functions including cell-cell adhesion, immune response to pathogens and...

s.

Eight collectins have been identified including mannan-binding lectin (MBL), surfactant protein A

Surfactant protein A

Surfactant protein A is an innate immune system collectin.- During Parturition :The role of Surfactant protein A in childbirth is indicated in studies with mice. Mice which gestate for 19 days typically show signs of SP-A in amniotic fluid at around 16 days. If SP-A is injected into the uterus at...

 (SP-A), surfactant protein D

Surfactant protein D

Surfactant, pulmonary-associated protein D, also known as SFTPD or SP-D, is a protein which in humans is encoded by the SFTPD gene.SFTPD is an innate immune system collectin.-Interactions:...

 (SP-D), collectin liver 1

Collectin liver 1

Collectin liver 1 is a collectin....

 (CL-L1), collectin placenta 1 (CL-P1), conglutinin

Conglutinin

Conglutinin is a collectin protein....

, collectin of 43 kDa

Collectin of 43 kDa

Collectin of 43 kDa is a collectin protein that acts as an antigen recognition protein. When an agent, zymosan, was injected into the tunicate Styela plicata , secretion of this collectin was tripled within 96 hours....

 (CL-43) and collectin of 46 kDa

Collectin of 46 kDa

Collectin of 46 kDa is a collectin protein. It has two cysteine residues on the N-terminal segment, a hydrophilic loop near the carbohydrate recognition domain's binding site, and a N-glycosylation site in the collagen region. It is expressed in bovine liver and thymus glands and binds to...

 (CL-46).

These molecules have been implicated as major modulators of the innate immune system

Innate immune system

The innate immune system, also known as non-specific immune system and secondary line of defence, comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner...

 where they have a key role in the first line of defense against invading microorganism

Microorganism

A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

s 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
  Cysteine
  Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
  
  -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 cytokine
  Cytokine
  Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...
  
  s, and reactive oxygen species
  Reactive oxygen species
  Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....
  
   in phagocyte
  Phagocyte
  Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek phagein, "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek kutos, "hollow vessel". They are...
  
  s by interacting with other cell surface receptors or by scavenging of bacterial molecules like lipopolysaccharide
  Lipopolysaccharide
  Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...
  
   (LPS).

• Some collectins (e.g. MBL) activate the lectin pathway of the complement system
  Complement system
  The complement system helps or “complements” the ability of antibodies and phagocytic cells to clear pathogens from an organism. It is part of the immune system called the innate immune system that is not adaptable and does not change over the course of an individual's lifetime...
  
  to increase membrane permeability of microorganisms and cause the destruction of that microbe.