Lectin Pathway of the Complement System

An Overview of Complement System

• The enormous complexity of the human immune system not only enables good defence against an astounding variety of pathogens, but also protects against an undesired response against self-components.
• The immune system is traditionally classified into two broad and interrelated sections, the innate and the adaptive. The innate immune system offers an instantaneous and non-specific initial line of defence via humoral, cellular, and mechanical processes, and plays a crucial role in pathogen challenge protection.
• The complement system is a crucial component of the innate immune system, having three roles that overlap: protection against infection, clearance of immune complexes and cell debris, and connection between innate and adaptive immunity.
• The complement system is comprised of over 35 plasma proteins, complement receptors on cell surfaces, and regulatory proteins. The vast majority of soluble proteins circulate as inactive proenzymes or zymogens.
• Inactive molecules become active upon proteolytic cleavage, resulting in a proteolytic cascade that induces several effector activities, such as phagocytosis, inflammation, cell lysis, and direction of the adaptive immune response.
• To protect host tissues from the potentially damaging effects of complement activation, many inhibitors closely regulate this system. Destroying invading pathogens and minimising damage to host cells and tissues are the two primary objectives of the complement system’s regulation and activation mechanisms.
• The disruption of this delicate equilibrium has detrimental repercussions on the host, with possibly dire consequences.
• On the surface of pathogens or damaged/infected cells, complement activation can occur via three separate but converging cascade pathways: classical, alternative, and lectin.
• Multiple inputs activate all three pathways independently, and the proteolytic cascades eventually converge on the activation of the main component C3, resulting in the construction of the membrane-attack complex (MAC).
• On immunological complexes, the activation of the classical pathway is begun by the binding of C1q to the Fc region of IgM or IgG.
• In contrast, the alternate route is activated by the spontaneous hydrolysis of C3 in plasma. Similarly to the alternate method, the lectin pathway can be activated without immune complexes.
• It is triggered by the binding of pattern-recognition plasma molecules such as mannose-binding lectin (MBL), collectin 11 (CL-K1), or ficolins to carbohydrates or acetylated residues present on microbes or to abnormal glycocalyx patterns on apoptotic, necrotic, or cancerous cells.
• Through the binding of MBL–MBL-associated serine proteases (MASPs) or ficolin–MASP complexes to fibrinogen or fibrin, the lectin pathway also plays a function in the coagulation system.

Lectin Pathway of Complement Activation

• The lectin pathway is activated by lectins, as its name suggests. Lectins are the proteins that bind to and recognise specific carbohydrate targets.
• The mannose-binding lectin (MBL) is one protein that participates in the complement activation lectin pathway.
• MBL is a large serum protein that binds to nonreduced mannose, fructose, and glucosamine on the mannose-containing polysaccharides of bacterial and other cell surfaces (mannans).
• MASP-1 and MASP-2 are two MBL-associated serine proteases that are secreted in response to MBL binding to a pathogen. MASP-1 and MASP-2 resemble C1r and C1s, respectively, but MBL resembles C1q.
• The formation of the MBL/MASP-1/MASP-2 complex leads to the activation of MASPs and the subsequent cleavage of C4 into C4a and C4b.
• In the same manner as the classical and alternative pathways, MAC is subsequently produced.

Components of Lectin Pathway

1. Mannose-binding lectin

• Mannose-binding lectin is a major recognition molecule in the lectin pathway, produced in liver cells and released as multimeric complexes with large molecular weight.
• It belongs to the collectin protein family, sharing collagen and carbohydrate-recognition domains (CRD).
• MBL is a C-type lectin due to its capacity to detect sugar moieties in a Ca2+-dependent manner; it is also referred to as “defensive collagen” due to its essential function in innate immunity and pathogen clearance.
• A trimer of identical polypeptide chains containing a cysteine-rich N-terminal domain, a collagen-like area, an alpha-helical coiled-coil neck domain, and a C-terminal CRD compose mannose-binding lectin.
• The disulfide bonds between the three chains constitute the structural unit of MBL, which polymerizes into higher-order MBL oligomers.
• Unlike dimers and higher-order oligomers, single MBL trimers are not fully functional, with tetramers predominating in circulation.
• The CRD domain is responsible for the recognition and binding of MBL to its ligands, and the oligomeric configuration confers multivalent and highly avid binding to targets.
• Mannose-binding lectin identifies repeating arrays of carbohydrate structures on pathogenic organisms such as viruses, bacteria, fungi, protozoa, multicellular parasites, and apoptotic/tumoral cells.
• MBL detects sugars with 3- and 4-OH groups positioned in the equatorial plane of the sugar ring, including glucose, L-fucose, N-acetylmannosamine (ManNAc), and N-acetylglucosamine (GlcNAc), but not galactose.
• MBL can also bind phospholipids, nucleic acids, and proteins that are not glycosylated.
• After binding to targets, MBL promotes multiple biological effects, including activation of the complement system via the lectin route, opsonophagocytosis, inflammatory regulation, and detection of changed self-structures.
• In addition, MBL may affect both mRNA and protein levels of cytokine production.
• By identifying damage-associated molecular patterns, mannose-binding lectin also contributes to the clearance of apoptotic cells (DAMPs).
• MBL enables the detection and phagocytosis of apoptotic cells by macrophages, resulting in their clearance, by binding to the terminal sugars of cytoskeletal proteins in apoptotic cells.
• The attachment of apoptotic cells to immature dendritic cells and macrophages is facilitated by both C1q and MBL.
• Defects in the clearance of apoptotic cells have been implicated in the aetiology of several autoimmune disorders, although the precise involvement of MBL in this process is uncertain.
• Experiments with MBL-deficient mice revealed an impairment in the elimination of apoptotic cells, but no association with autoimmune illness.

Structural subunits of mannan-binding lectin (MBL) and ficolins

• MBL and ficolins both have a brief cysteine-rich region at the N-terminus, followed by a collagen-like sequence [length provided in number of amino acids (aa)].
• The C-terminal region consists of a carbohydrate-recognition binding domain for MBL (shown by ovals) and a fibrinogen-like domain for ficolins (shown as tulip forms).
• The polypeptides bind via their collagen-like region to produce triple helices (trimers), which then build higher oligomeric structures (tetramers to hexamers).
• Even though they have vastly different structures, ficolin polypeptides and trimers interact similarly to MBL, generating highly oligomeric forms (tetramers).
• MBL-associated serine proteases interact with collagen-like regions, triggering the complement lectin pathway.

2. MBL serum levels and MBL2 gene polymorphisms

• Mannose-binding lectin is encoded by the MBL2 gene, which is located on chromosome 10’s long arm (10q11.2–q21).
• It is considered an acute-phase reactant, and its levels can increase up to threefold during the acute phase of an immune response, primarily due to the upregulation of acute-phase mediators.
• Serum concentrations of MBL range from a few nanograms per millilitre to greater than 10 g/ml, with a typical value of approximately 0.8 g/ml.
• However, MBL levels are predominantly determined by MBL2 genetic polymorphisms, which account for up to 10-fold inter-individual differences in circulating MBL levels. In addition to genetic variance, MBL levels may also vary considerably over the course of a lifetime.
• Mannose-binding lectin insufficiency is quite prevalent, affecting 8–10% of the population and often characterised as 100 ng/ml in the circulation.
• MBL deficiency is more dangerous when other immunological abnormalities are present, while the majority of MBL-deficient individuals are otherwise healthy.
• MBL deficiency has been linked to upper respiratory tract infections in young infants and to an increased risk of serious infections in chemotherapy patients.
• Infections caused by intracellular pathogens, such as Mycobacterium spp. and Leishmania chagasi, which use C3 opsonization and C3 receptors to penetrate host cells, it may be advantageous.
• MBL2 is a highly polymorphic gene whose variants account for substantial differences in MBL levels and functional activity.
• These variants consist of at least one synonymous SNP (on codon 44 for asparagine) and eight non-synonymous variants situated in the first exon of the MBL2 gene (including B, C, and D, which are detailed in the next paragraph).
• MBL2*H and L alleles (due to a polymorphism at 550 bp), X and Y alleles (due to an SNP at 221 bp), and P and Q alleles (due to a non-coding SNP at +4 bp) also influence MBL levels.
• Sumiya et al. mapped the entire MBL2 gene in three British youngsters with recurrent bacterial infections and low MBL levels in 1991. Everyone carried the B allele (an exon 1 point mutation at codon 54, changing GGC to GAC and causing an amino acid change of glycine to aspartic acid – p.Gly54Asp).
• Others subsequently discovered two additional deficiency-causing frequent substitutions: allele D in codon 52 (CGT to TGT), which changes arginine to cysteine (p.Arg52Cys), and allele C in codon 57 (GGA to GAA), which replaces glycine with glutamic acid (p.Gly57Glu).
• In homozygous (e.g., B/B) or compound heterozygous (e.g., B/C) carriers, Exon 1 mutations drastically impair protein assembly and stability, increasing the proportion of weakly oligomerized MBL with diminished complement activation and ligand binding capacity.
• The wild allele at these locations is denoted by the letter A, while the collective designation for the D, B, and C alleles is 0.
• While 0/0 individuals have amounts of high-order MBL oligomers that are nearly undetectable, A/0 individuals may have lower plasma protein levels.
• In addition, a promoter mutation with X and Y alleles 221 kb before the transcription start site (g.602G > C) significantly reduces the amounts of otherwise completely functioning MBL proteins.

3. Ficolins

• Similarly to MBL, ficolins are pattern-recognition receptors that can bind to MASPs and activate the complement system via the lectin route; they play a crucial role in the immune response against clinically significant infections.
• In addition to activating complement, they limit infection by promoting macrophage secretion of interferon gamma (IFN-), interleukin-17 (IL-17), interleukin-6 (IL-6), tumour necrosis factor alpha (TNF-), and nitric oxide (NO).
• Similar to MBL, ficolins form oligomers of four structural subunits connected by disulfide bridges at the N-terminal regions, but higher or smaller oligomers appear to be less prevalent for ficolins.
• They should not be referred to as lectins, as they target acetylated molecules relatively independently of the structure of the acetylated molecule (carbohydrates being the preferred ligands for lectins). There are three human ficolins, each of which is encoded by its own gene.

Ficolin-1

• Ficolin-1 is linked to cell membranes or soluble in plasma at concentrations between 0.05 and 1.0 g/ml.
• It is also detected in monocyte secretory granules, neutrophil gelatinase granules, and type II alveolar epithelial cells.
• Ficolin-1 identifies common acetylated chemicals, such as GlcNAc and GalNAc, which bind to multiple Gram-positive (Staphylococcus aureus) and Gram-negative bacteria (Salmonella typhimurium LT2).
• It is the only human ficolin capable of binding to sialic acid, which is present on the surface of pathogens such as Streptococcus agalactiae and immune cells.
• Ficolin-1 is therefore believed to play a function in the regulation of immune cell interactions, blood coagulation, and/or fibrinolysis.
• Significantly, Ficolin-1 and pentraxin-3 heterocomplexes operate as non-inflammatory signals, boosting the clearance of changed self-cells and influencing the generation of IL-8.
• The FCN1 gene is on chromosome 9q34 and is composed of nine exons. At least eight SNPs described for the FCN1 gene are related with Ficolin-1 levels, four of which are located in the gene’s promoter and first exon.
• These polymorphisms contribute to the considerable variation (up to 15-fold) in Ficolin-1 plasma concentrations between individuals.
• FCN1 polymorphisms were related with an increased risk of death in patients with systemic inflammation and with rheumatoid arthritis susceptibility.
• Low levels of Ficolin-1 have been linked to a 12-fold greater risk of deadly necrotizing enterocolitis, the requirement for mechanical ventilation, and the development of serious infections in chemotherapy-treated cancer patients.

Ficolin-2

• Ficolin-2 is a plasma protein that is produced mostly in the liver, although low mRNA levels have also been detected in the bone marrow, gut, tonsils, and foetal lung.
• Ficolin-2 can bind N-acetylated molecules, such as Acetyl-d-glucosamine (GlcNAc), N-acetylgalactosamine, and N-acetyllactosamine, along with artificially acetylated substances.
• It also binds N-acetylneuraminic acid found on encapsulated opportunistic pathogens such as Group B streptococci (Streptococcus agalactiae), bacterial peptidoglycan (PGN), fungal 1,3-beta-D-glucan, and hepatitis C virus envelope glycoproteins.
• Ficolin-2 binds to Mycobacterium bovis and flagellated protozoa, such as Giardia intestinalis and Trypanosoma cruzi, and interacts with C-reactive protein, consolidating its binding to bacteria and activating complement.
• Three SNPs in the promoter region and one in exon 8 are associated with variance in Ficolin-2 plasma levels: 986G > A, 602G > A, and 4A > G and p.Ala258Ser. Two other SNPs, at locations 558 and 64, do not appear to alter gene expression.
• Ficolin-2 deficiency (1,200 ng/ml) has been associated with bronchiectasis and respiratory infection, particularly in the presence of atopy (94–96), but did not influence susceptibility to invasive pneumococcal illness.
• In neonates from Poland, low Ficolin-2 levels were also associated with prematurity, low birth weight, and perinatal infections, as well as susceptibility to chronic Chagas disease.
• And while not being connected with the development of malaria, children with severe malaria had higher levels of Ficolin-2 than those with moderate malaria.
• In contrast, FCN2 SNPs associated with normal Ficolin-2 levels provided protection against leprosy susceptibility.

Ficolin-3

• Ficolin-3 is the most abundant recognition molecule of the lectin-pathway, with a mean plasma concentration between 3 and 54 g/ml and a 10-fold variation across people.
• Ficolin-3 was shown to be significantly expressed in liver and lung tissues, demonstrating its importance in lectin pathway activation and pulmonary host defence.
• Ficolin-3 is therefore believed to play a significant role in both systemic and local innate immune responses.
• Numerous bacteria, including Salmonella typhimurium, Salmonella minnesota, Escherichia coli, and Aerococcus viridans, include acetyl groups. Ficolin-3 has been demonstrated to identify these acetyl groups.
• It was also discovered that it shares binding sites with Ficolin-2 and MBL on the Giardia intestinalis surface.
• In addition, Ficolin-3 may facilitate the elimination of late apoptotic cells and may have a protective effect against autoimmunity.
• The human FCN3 gene is located on 1p36.11 and is highly conserved. Five amino acid exchanges with allele frequencies of less than 5% were described: p.Leu12Val, p.Leu117fs, p.Thr125Ala, p.Glu166Asp, and p.Val287Ala.
• This high level of conservation suggests that Ficolin-3 may play a vital role in the immunological response. Indeed, Ficolin-3 deficiency is exceedingly uncommon and has been linked to necrotizing enterocolitis in premature infants.

4. MBL-associated serine proteases

• On binding of MBL, ficolins, and CL-K1 to carbohydrates or acetyl groups on the surface of pathogens or changed self-tissues, MBL-associated serine proteases serve as activators of the lectin pathway.
• Five proteins have been discovered thus far, including three MASP enzymes (MASP-1, MASP-2, and MASP-3) and two truncated proteins, MAp19 and MAp44, which lack the serine protease domain and, hence, functional activity.
• In the presence of Ca2+, all MASPs are capable of interacting with MBL, ficolins, and CL-K1 to create a proteolytic complex.
• Both MASP-1 and MASP-2 are essential for activating the lectin pathway. Recent research demonstrated that MASP-1 activation can trigger MASP-2 activation. MASP-2 is capable of autoactivation, although under physiological conditions, MASP-1 is the essential activator of MASP-2.
• MASP-2 is a protease that efficiently cleaves C4 and C2 to generate C3 convertase (113, 117). Due to competition for MASP binding sites on recognition molecules, MASP-3 appears to reduce the lectin-activity. pathway’s
• Additionally, MASP-3 is mainly complexed with Ficolin-3 and is believed to block complement activation mediated by Ficolin-3.
• MASP-3 also participate on developmental processes. MAp44 has been demonstrated to adversely control the lectin pathway by competing for the same binding sites as MASP-2 and MASP-1. The roles of MAp19 and MAp44 are currently poorly known, although MAp44 has been shown to negatively regulate the lectin pathway.
• MASP-1 was the first reported MASP. While MASP-1 and MASP-2 are primarily produced in the liver and are present in plasma at concentrations of 11 and 0.4 g/ml, respectively, MASP-3 is produced in multiple different tissues in addition to the liver.
• Both C1r and C1s share structural similarities with all three MASPs. MASP-1, MASP-3, and MAp44 are encoded by the MASP1 gene situated on chromosome 3q27–q28, whereas MASP-2 and MAp19 are encoded by the MASP2 gene on chromosome 1p36.23–31.
• Some MASP1 and MASP2 gene polymorphisms result in altered blood levels and functions of MASPs, consequently affecting complement activation via the lectin route.
• Ammitzbll et al. discovered 10 SNPs related with blood levels of MASP-1, MASP-3, and MAp44 in the MASP1 gene.
• In cystic fibrosis patients, MASP1 SNPs were related with 3MC syndrome (131–133) and Pseudomonas aeruginosa colonisation.
• In addition, MASP levels (MASP-1, MASP-2, and MASP-3) were found to be predictive of infection and prolonged intensive care reliance in critically unwell children.
• In contrast, MASP2 polymorphisms were related with leprosy, human T lymphotropic virus infection, malaria, Chagas disease, bacterial infections, and hepatitis C susceptibility.
• Several disorders, including schizophrenia, septic shock, acute lymphoblastic leukaemia, non-Hodgkin lymphoma, central nervous system malignancies, and colorectal cancer, have been linked to MASP-2 levels.
• Collectively, these investigations have provided evidence that MASPs have a growing and significant biological function in human illnesses.

Steps of Lectin Pathway of Complement Activation

• When pattern-recognition molecules (MBL, CL-K1, and ficolins) bind to pathogen-associated molecular patterns (PAMPs) (D-mannose, N-acetyl-D-glucosamine, or acetyl groups) on the surface of pathogens or apoptotic or necrotic cells, the lectin pathway is activated.
• MBL, CL-K1, and ficolins in circulation form complexes with MASP-1 and MASP-2 dimers.
• MASP-1 can auto-activate and activate MASP-2 (10) following the binding of MBL/MASPs, CL-K1/MASPs, or ficolin/MASPs complexes to their targets, resulting in C4 and C2 cleavage.
• This permits the assembly of the C3 and C5 convertases, followed by the activation of C3 and C5, respectively, and the production of C3a and C5a, two pro-inflammatory anaphylatoxins that augment the inflammatory response.
• The fragment C3b binds covalently to hydroxyl and amino groups on the surface of all three routes’ target molecules.
• In the absence of complement regulatory proteins, the alternative pathway results in a substantial increase in the quantity of surface-bound C3b molecules.
• In this amplification loop, factor B binds to the connected C3b and is cleaved by factor D, resulting in the alternative pathway C3 convertase C3bBb, which speeds up C3b production.
• C3b identifies antigens/pathogens for opsonization, antigen presentation, or death by phagocytes through interacting with complement membrane receptors CR1, CR2, CR3, and CR4, as well as the immunoglobulin superfamily member CRIg.
• The complement cascade concludes with the development of the multiprotein complex (C5b, C6, C7, C8, and C9n) known as the terminal complement complex or MAC, which are inserted as pores up to 11 nm in size into the cell membrane, resulting in loss of membrane integrity and ultimately cell death.

Deficiencies of the Lectin Pathway Components

MBL, M-ficolin, L-ficolin, H-ficolin, CL-11, MASPs

• Manose Binding Lectin Deficiency Does Not Constitute a Primary Immunodeficiency MBL (manose-binding lectin) is a component of the complement system’s lectin pathway, one of numerous immune defence components.
• The lectin pathway may be the first to respond before to a conventional immune response. Some examples of increased susceptibility to bacterial infection were assumed to be attributable to MBL deficiency.
• However, when a test was created to quantify MBL in the blood, it was revealed that low or nonexistent MBL is quite widespread, affecting between 5 and 30% of the population.
• Therefore, its absence alone cannot be a cause of severe immunodeficiency; otherwise, a huge section of the world’s population would suffer from frequent and recurring infections that might be fatal.
• Unfortunately, the MBL test is still infrequently ordered during immunodeficiency evaluations, and a low or nonexistent MBL level is incorrectly taken as evidence of a primary immunodeficiency condition.
• Expert immunologists with experience caring for PI patients, on the other hand, feel that low or absent components of this lectin system, particularly low or absent MBL, do not cause immunodeficiency on their own.
• There is no suggested treatment for low or nonexistent MBL, nor is immunoglobulin replacement therapy appropriate for this condition.
• It is essential to emphasise that the detection of a low or missing MBL does not suggest that the source of an individual’s infections has been identified, and that the diagnostic procedure must continue until the proper diagnosis is made.
• In such instances, the IDF suggests consulting an experienced immunologist for assistance with the diagnostic examination.

References

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