Pattern recognition receptor
Encyclopedia
Pattern recognition receptors (PRRs) are a primitive part of the immune system
. They are protein
s expressed by cells of the innate immune system
to identify pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens or cellular stress, as well as damage-associated molecular patterns (DAMPs), which are associated with cell components released during cell damage. They are also called pathogen recognition receptors; and primitive pattern recognition receptors because they are an early part of the immune system to evolve, before adaptive immunity.
s (PAMPs) and include bacterial carbohydrates (such as lipopolysaccharide
or LPS, mannose
), nucleic acids (such as bacterial or viral DNA or RNA), bacterial peptides (flagellin, ax21), peptidoglycan
s and lipoteichoic acid
s (from Gram positive bacteria), N-formylmethionine
, lipoprotein
s and fungal glucan
s.
Endogenous stress signals are called danger-associated molecular patterns (DAMPs) and include uric acid
.
specificity, function, localization and/or evolutionary relationships. On the basis of function, PRRs may be divided into endocytic PRRs or signaling PRRs.
s (TLRs). Toll-like receptors were first discovered in Drosophila
and trigger the synthesis and secretion of cytokine
s and activation of other host defense programs that are necessary for innate or adaptative immune responses. TLRs have been found in many species. In mammals, these receptors have been assigned numbers 1 to 11 (TLR1-TLR11). Interaction of TLRs with their specific PAMP induces NF-κB signaling and the MAP kinase
pathway and therefore the secretion of pro-inflammatory cytokine
s and co-stimulatory molecule
s. Molecules released following TLR activation signal to other cells of the immune system making TLRs key elements of innate immunity and adaptive immunity.
(MR) is a PRR primarily present on the surface of macrophage
s and dendritic cell
s. The MR belongs to the multilectin receptor protein group and, like the TLRs, provides a link between innate and adaptive immunity. It recognizes and binds to repeated mannose units on the surfaces of infectious agents and its activation triggers endocytosis and phagocytosis of the microbe via the complement system. Specifically, mannose binding triggers recruitment of MBL-associated serine proteases (MASPs). The serine proteases activate themselves in a cascade, amplifying the immune response: MBL interacts with C4, binding the C4b subunit and releasing C4a into the bloodstream; similarly, binding of C2 causes release of C2a. Together, MBL, C4b and C2b are known as the C3 convertase. C3 is cleaved into its a and b subunits, and C3b binds the convertase. These together are called the C5 convertase. Similarly again, C5b is bound and C5a is released. C5b recruits C6, C7, C8 and multiple C9s. C5, C6, C7, C8 and C9 form the membrane attack complex (MAC).
The NOD-like receptors (NLRs) are cytoplasmic proteins that regulate inflammatory and apoptotic responses. Approximately 20 of these proteins have been found in the mammalian genome and include two major subfamilies called NODs and NALPs, the MHC Class II
transactivator (CIITA
), IPAF, BIRC1, and other molecules. This family of proteins is greatly expanded in plants, and constitutes a core component of plant immune systems
. Some of these proteins recognize endogenous or microbial molecules or stress responses and form oligomers that, in animals, activate inflammatory caspases (e.g. caspase 1
) causing cleavage and activation of important inflammatory cytokine
s such as IL-1
, and/or activate the NF-κB signaling pathway to induce production of inflammatory molecules. The NLR family is known under several different names, including the CATERPILLER (or CLR) or NOD-LRR family.
has been shown to be mediated by a group of RNA Helicases which in turn recruit factors via twin N-terminal CARD domain
s to activate antiviral gene programs. Three such helicases have been described in mammals—RIG-I and MDA5 (recognizing 5'triphosphate-RNA and dsRNA, respectively), which activate antiviral signaling , and LGP2, which appears to act as a dominant-negative inhibitor.
also encode resistance proteins that resemble NOD-like receptors (see above), that feature NBS and LRR domains and can also carry other conserved interaction domains such as the TIR cytoplasmic domain found in Toll and Interleukin Receptors. The NBS-LRR proteins are required for effector triggered immunity (ETI).
s, pentraxin proteins such as serum amyloid
and C-reactive protein
, lipid transferases, peptidoglycan recognition proteins (PGRs) and the LRR, XA21D are all secreted proteins. One very important collectin is mannan-binding lectin
(MBL), a major PRR of the innate immune system that binds to a wide range of bacteria, viruses, fungi and protozoa. MBL predominantly recognizes certain sugar groups on the surface of microorganisms but also binds phospholipid
s, nucleic acid
s and non-glycosylated
proteins.
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
. They are protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s expressed by cells 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...
to identify pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens or cellular stress, as well as damage-associated molecular patterns (DAMPs), which are associated with cell components released during cell damage. They are also called pathogen recognition receptors; and primitive pattern recognition receptors because they are an early part of the immune system to evolve, before adaptive immunity.
Molecules recognized
The microbe-specific molecules that are recognized by a given PRR are called pathogen-associated molecular patternPathogen-associated molecular pattern
Pathogen-associated molecular patterns, or PAMPs, are molecules associated with groups of pathogens, that are recognized by cells of the innate immune system. These molecules can be referred to as small molecular motifs conserved within a class of microbes...
s (PAMPs) and include bacterial carbohydrates (such as 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...
or LPS, mannose
Mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....
), nucleic acids (such as bacterial or viral DNA or RNA), bacterial peptides (flagellin, ax21), peptidoglycan
Peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
s and lipoteichoic acid
Lipoteichoic acid
Lipoteichoic acid is a major constituent of the cell wall of Gram-positive bacteria. These organisms have an inner membrane and, external to it, a thick peptidoglycan layer. It consists of teichoic acids, long chains of ribitol phosphate and is anchored to the lipid bilayer via a glyceride...
s (from Gram positive bacteria), N-formylmethionine
N-Formylmethionine
N-Formylmethionine is a proteinogenic amino acid found in Bacteria and related Prokaryotic organelles. It is a derivative of the amino acid methionine in which a formyl group has been added to the amino group...
, lipoprotein
Lipoprotein
A lipoprotein is a biochemical assembly that contains both proteins and lipids water-bound to the proteins. Many enzymes, transporters, structural proteins, antigens, adhesins, and toxins are lipoproteins...
s and fungal glucan
Glucan
A glucan molecule is a polysaccharide of D-glucose monomers linked by glycosidic bonds.Many beta-glucans are medically important.-Types:The following are glucans:-Alpha:...
s.
Endogenous stress signals are called danger-associated molecular patterns (DAMPs) and include uric acid
Uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...
.
Classification
PRRs are classified according to their ligandLigand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
specificity, function, localization and/or evolutionary relationships. On the basis of function, PRRs may be divided into endocytic PRRs or signaling PRRs.
- Signaling PRRs include the large families of membrane-bound Toll-like receptors and cytoplasmic NOD-like receptors.
- Endocytic PRRs promote the attachment, engulfment and destruction of microorganisms by phagocytes, without relaying an intracellular signal. These PRRs recognize carbohydrates and include mannose receptors of macrophageMacrophageMacrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
s, glucanGlucanA glucan molecule is a polysaccharide of D-glucose monomers linked by glycosidic bonds.Many beta-glucans are medically important.-Types:The following are glucans:-Alpha:...
receptors present on all phagocytes and scavenger receptorScavenger receptorScavenger receptors are a group of receptors that recognize modified low-density lipoprotein by oxidation or acetylation. This naming is based on a function of cleaning : Scavenger receptors widely recognize and uptake macromolecules having a negative charge as well as modified LDL.-Function:It is...
s that recognize charged ligands, are found on all phagocytes and mediate removal of apoptotic cells.
Receptor kinases
PRRs were first discovered in plants. Since that time many plant PRRs have been predicted by genomic analysis (370 in rice; 47 in Arabidopsis). Unlike animal PRRs, which associated with intracellular kinases via adaptor proteins (see non-RD kinases below), plant PRRs are composed of an extracellular domain, transmembrane domain, juxtamembrane domain and intracellular kinase domain as part of a single protein.Toll-like receptors
Recognition of extracellular or endosomal pathogen-associated molecular patterns is mediated by transmembrane proteins known as toll-like receptorToll-like receptor
Toll-like receptors are a class of proteins that play a key role in the innate immune system. They are single, membrane-spanning, non-catalytic receptors that recognize structurally conserved molecules derived from microbes...
s (TLRs). Toll-like receptors were first discovered in Drosophila
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
and trigger the synthesis and secretion of 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 activation of other host defense programs that are necessary for innate or adaptative immune responses. TLRs have been found in many species. In mammals, these receptors have been assigned numbers 1 to 11 (TLR1-TLR11). Interaction of TLRs with their specific PAMP induces NF-κB signaling and the MAP kinase
Mitogen-activated protein kinase
Mitogen-activated protein kinases are serine/threonine-specific protein kinases that respond to extracellular stimuli and regulate various cellular activities, such as gene expression, mitosis, differentiation, proliferation, and cell survival/apoptosis.-Activation:MAP kinases are activated...
pathway and therefore the secretion of pro-inflammatory 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 co-stimulatory molecule
Co-stimulation
During the activation of lymphocytes, co-stimulation is often crucial to the development of an effective immune response. Co-stimulation is required in addition to the antigen-specific signal from their antigen receptors.- Co-stimulation T cells require :...
s. Molecules released following TLR activation signal to other cells of the immune system making TLRs key elements of innate immunity and adaptive immunity.
The mannose receptor
The mannose receptorMannose receptor
The mannose receptor is a C-type lectin carbohydrate binding protein primarily present on the surface of macrophages and dendritic cells. It helps recognize pathogens that have mannose on their surface, and triggers one pathway of the complement system....
(MR) is a PRR primarily present on the surface of macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
s and dendritic cell
Dendritic cell
Dendritic cells are immune cells forming part of the mammalian immune system. Their main function is to process antigen material and present it on the surface to other cells of the immune system. That is, dendritic cells function as antigen-presenting cells...
s. The MR belongs to the multilectin receptor protein group and, like the TLRs, provides a link between innate and adaptive immunity. It recognizes and binds to repeated mannose units on the surfaces of infectious agents and its activation triggers endocytosis and phagocytosis of the microbe via the complement system. Specifically, mannose binding triggers recruitment of MBL-associated serine proteases (MASPs). The serine proteases activate themselves in a cascade, amplifying the immune response: MBL interacts with C4, binding the C4b subunit and releasing C4a into the bloodstream; similarly, binding of C2 causes release of C2a. Together, MBL, C4b and C2b are known as the C3 convertase. C3 is cleaved into its a and b subunits, and C3b binds the convertase. These together are called the C5 convertase. Similarly again, C5b is bound and C5a is released. C5b recruits C6, C7, C8 and multiple C9s. C5, C6, C7, C8 and C9 form the membrane attack complex (MAC).
NOD-like receptors
For more details, see NOD-like receptor.The NOD-like receptors (NLRs) are cytoplasmic proteins that regulate inflammatory and apoptotic responses. Approximately 20 of these proteins have been found in the mammalian genome and include two major subfamilies called NODs and NALPs, the MHC Class II
MHC class II
MHC Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells ....
transactivator (CIITA
CIITA
CIITA is a human gene which encodes a protein called the class II, major histocompatibility complex, transactivator. Mutations in this gene are responsible for the bare lymphocyte syndrome in which the immune system is severely compromised and cannot effectively fight infection...
), IPAF, BIRC1, and other molecules. This family of proteins is greatly expanded in plants, and constitutes a core component of plant immune systems
Plant disease resistance
Plant disease resistance is crucial to the reliable production of food, and it provides significant reductions in agricultural use of fuel, land, water and other inputs. There are numerous examples of devastating plant disease impacts , as well as recurrent severe plant disease issues...
. Some of these proteins recognize endogenous or microbial molecules or stress responses and form oligomers that, in animals, activate inflammatory caspases (e.g. caspase 1
Caspase 1
Caspase 1 is an enzyme that proteolytically cleaves other proteins, such as the precursor forms of the inflammatory cytokines interleukin 1-β and interleukin 18, into active mature peptides...
) causing cleavage and activation of important inflammatory 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 such as IL-1
IL-1
IL-1 may refer to:* Interleukin 1, a protein* Illinois' 1st congressional district* Illinois Route 1* Building 1 of Infinite Loop , the Headquarters of Apple Inc....
, and/or activate the NF-κB signaling pathway to induce production of inflammatory molecules. The NLR family is known under several different names, including the CATERPILLER (or CLR) or NOD-LRR family.
-
- NODs
- The ligands are currently known for NOD1NOD1Nucleotide-binding oligomerization domain-containing protein 1 is a protein receptor that in humans is encoded by the NOD1 gene. It recognizes bacterial molecules and stimulates an immune reaction....
and NOD2NOD2Nucleotide-binding oligomerization domain-containing protein 2 also known as caspase recruitment domain-containing protein 15 or inflammatory bowel disease protein 1 is a protein that in humans is encoded by the NOD2 gene located on chromosome 16. NOD2 plays an important role in the immune system...
. NOD1 recognizes a molecule called meso-DAP, which is a peptidoglycanPeptidoglycanPeptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
constituent only of Gram negative bacteria. NOD2 proteins recognize intracellular MDP (muramyl dipeptide), which is a peptidoglycan constituent of both Gram positive and Gram negative bacteria. NODs transduce signals in the pathway of NF-κB and MAP kinases via the serine-threonine kinase called RIP2. NOD proteins are so named because they contain a nucleotide-binding oligomerization domain which binds nucleoside triphosphateNucleoside triphosphateNucleoside triphosphate is a nucleoside with three phosphates. Natural nucleoside triphosphates include adenosine triphosphate , guanosine triphosphate , cytidine triphosphate , 5-methyluridine triphosphate , and uridine triphosphate . These terms refer to those nucleoside triphosphates that...
. NODs signal via N-terminal CARD domainCARD domainCaspase recruitment domains, or Caspase activation and recruitment domains , are interaction motifs found in a wide array of proteins, typically those involved in processes relating to inflammation and apoptosis. These domains mediate the formation of larger protein complexes via direct...
s to activate downstream gene induction events, and interact with microbial molecules by means of a C-terminal leucine-rich repeatLeucine-rich repeatA leucine-rich repeat is a protein structural motif that forms an α/β horseshoe fold. It is composed of repeating 20–30 amino acid stretches that are unusually rich in the hydrophobic amino acid leucine...
(LRR) region.
-
- NALPs
- Like NODs, these proteins contain C-terminal LRRs, which appear to act as a regulatory domain and may be involved in the recognition of microbial pathogens. Also like NODs, these proteins also contain a nucleotide binding site (NBS) for nucleoside triphosphates. Interaction with other proteins (e.g. the adaptor molecule ASCPYCARDApoptosis-associated speck-like protein containing a CARD or ASC is a protein that in humans is encoded by the PYCARD gene.This gene encodes an adaptor protein that is composed of two protein–protein interaction domains: a N-terminal PYRIN-PAAD-DAPIN domain and a C-terminal caspase-recruitment...
) is mediated via N-terminal pyrin (PYD) domain. There are 14 members of this subfamily in humans (called NALP1 to NALP14). Mutations in NALP3 are responsible for the autoinflammatory diseases familial cold autoinflammatory syndrome, Muckle-Wells syndromeMuckle-Wells syndromeMuckle–Wells syndrome , also known as , is a rare autosomal dominant disease which causes sensorineural deafness, recurrent hives, and can lead to amyloidosis. Individuals with MWS often have episodic fever, chills, and painful joints. As a result, MWS is considered a type of periodic fever syndrome...
and neonatal onset multisystem inflammatory diseaseNeonatal onset multisystem inflammatory diseaseNeonatal onset multisystem inflammatory disease is a rare genetic periodic fever syndrome which causes uncontrolled inflammation in multiple parts of the body starting in the newborn period...
. Activators of NALP3 include muramyl dipeptideMuramyl dipeptideMuramyl dipeptide is a peptidoglycan constituent of both Gram positive and Gram negative bacteria. It is composed of N-acetylmuramic acid linked by its lactic acid moiety to the N-terminus of an L-alanine D-isoglutamine dipeptide.-See also:* Dipeptide...
, bacterial DNA, ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
, toxins, double stranded RNARNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, paramyxovirusParamyxovirusParamyxoviruses are viruses of the Paramyxoviridae family of the Mononegavirales order; they are negative-sense single-stranded RNA viruses responsible for a number of human and animal diseases.-Genera:*Subfamily Paramyxovirinae**Genus Avulavirus Paramyxoviruses (from Greek para-, beyond, -myxo-,...
es and uric acidUric acidUric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...
crystals. Although these specific molecules have been shown to activate NALP3, it remains unclear whether this is due to direct binding or due to cellular stress induced by these agents.
-
- Other NLRs
- Other NLRs such as IPAF and NAIP5/Birc1e have also been shown to activate caspase-1 in response to SalmonellaSalmonellaSalmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...
and LegionellaLegionellaLegionella is a pathogenic Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila. It may be readily visualized with a silver stain....
.
RNA Helicases
Intracellular recognition of viral double-stranded (ds) and single stranded RNARNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
has been shown to be mediated by a group of RNA Helicases which in turn recruit factors via twin N-terminal CARD domain
CARD domain
Caspase recruitment domains, or Caspase activation and recruitment domains , are interaction motifs found in a wide array of proteins, typically those involved in processes relating to inflammation and apoptosis. These domains mediate the formation of larger protein complexes via direct...
s to activate antiviral gene programs. Three such helicases have been described in mammals—RIG-I and MDA5 (recognizing 5'triphosphate-RNA and dsRNA, respectively), which activate antiviral signaling , and LGP2, which appears to act as a dominant-negative inhibitor.
Plant PRRs
Plants contain a significant number of PRRs that share remarkable structural and functional similarity with drosophila TOLL and mammalian TLRs. The first PRR identified in plants or animals was the XA21 protein, conferring resistance to the Gram-negative bacterial pathogen Xanthomonas oryzae pv. oryzae. Since that time two other plants PRRs, Arabidopsis FLS2 (flagellin) and EFR (elongation factor Tu receptor)have been isolated. The corresponding PAMPs for XA21, FLS2 and EFR have all been identified. Upon ligand recognition, the plant PRRs transduce "PAMP-triggered immunity" (PTI). Plant immune systemsPlant disease resistance
Plant disease resistance is crucial to the reliable production of food, and it provides significant reductions in agricultural use of fuel, land, water and other inputs. There are numerous examples of devastating plant disease impacts , as well as recurrent severe plant disease issues...
also encode resistance proteins that resemble NOD-like receptors (see above), that feature NBS and LRR domains and can also carry other conserved interaction domains such as the TIR cytoplasmic domain found in Toll and Interleukin Receptors. The NBS-LRR proteins are required for effector triggered immunity (ETI).
NonRD kinases
PRRs commonly associate with or contain members of a monophyletic group of kinases called the interleukin-1 receptor-associated kinase (IRAK) family that include Drosophila Pelle, human IRAKs, rice XA21 and Arabidopsis FLS2. In mammals, PRRs can also associate with members of the receptor-interacting protein (RIP) kinase family, distant relatives to the IRAK family. Some IRAK and RIP family kinases fall into a small functional class of kinases termed non-RD, many of which do not autophosphorylate the activation loop. A survey of the yeast, fly, worm, human, Arabidopsis, and rice kinomes (3,723 kinases) revealed that despite the small number of non-RD kinases in these genomes (9%–29%), 12 of 15 kinases known or predicted to function in PRR signaling fall into the non-RD class. In plants, all PRRs characterized to date belong to the non-RD class. These data indicate that kinases associated with PRRs can largely be predicted by the lack of a single conserved residue and reveal new potential plant PRR subfamilies.Secreted PRRs
A number of PRRs do not remain associated with the cell that produces them. Complement receptors, collectinCollectin
Collectins are soluble pattern recognition receptors belonging to the superfamily of collagen containing C-type lectins.Eight collectins have been identified including mannan-binding lectin , surfactant protein A , surfactant protein D , collectin liver 1 , collectin placenta 1 , conglutinin,...
s, pentraxin proteins such as serum amyloid
Amyloid
Amyloids are insoluble fibrous protein aggregates sharing specific structural traits. Abnormal accumulation of amyloid in organs may lead to amyloidosis, and may play a role in various neurodegenerative diseases.-Definition:...
and C-reactive protein
C-reactive protein
C-reactive protein is a protein found in the blood, the levels of which rise in response to inflammation...
, lipid transferases, peptidoglycan recognition proteins (PGRs) and the LRR, XA21D are all secreted proteins. One very important collectin is mannan-binding lectin
Mannan-binding lectin
Mannose-binding lectin , also named mannose- or mannan-binding protein , is an important factor in innate immunity.-Function:MBL belongs to the class of collectins in the C-type lectin superfamily, whose function appears to be pattern recognition in the first line of defense in the pre-immune...
(MBL), a major PRR of the innate immune system that binds to a wide range of bacteria, viruses, fungi and protozoa. MBL predominantly recognizes certain sugar groups on the surface of microorganisms but also binds phospholipid
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
s, nucleic acid
Nucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...
s and non-glycosylated
Glycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
proteins.