Complement System

The complement system is a sophisticated part of the immune system that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells, promotes inflammation, and attacks the pathogen's cell membrane. This system is named for its capacity to "complement" the killing of pathogens by antibodies.

Comprising over 30 small proteins produced by the liver, the complement system circulates in the blood serum and within tissues throughout the body. These proteins are normally inactive but become sequentially activated in response to a pathogen. Activation can occur via three pathways: the classical pathway, which is triggered by antibodies bound to a pathogen; the lectin pathway, which is activated by mannose-binding lectin attaching to the pathogen surface; and the alternative pathway, which can be initiated directly by pathogen surfaces.

Each pathway leads to the cleavage of the protein C3, generating C3a and C3b fragments. C3b binds to the pathogen surface and can lead to direct opsonization—tagging the pathogen for destruction by phagocytes. C3a, along with other small fragments released during complement activation, acts as a chemoattractant to recruit immune cells to the site of infection and promote inflammation.

The final common step of the complement activation pathways is the assembly of the membrane attack complex (MAC). This complex forms a pore in the membrane of the target cell, leading to cell lysis and death. The MAC is particularly effective against gram-negative bacteria.

Regulation of the complement system is crucial, as overactivation can damage host cells. Regulators are present in blood plasma and on cell surfaces, ensuring the system is directed only at pathogens or damaged cells.

The complement system plays a crucial role not only in innate immunity but also as a bridge to adaptive immunity. It assists in clearing immune complexes and apoptotic cells, and in enhancing the humoral response and immune memory.

Understanding and manipulating the complement system has therapeutic potential for numerous diseases, including those that involve excessive or inappropriate activation of the complement pathways.

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  1. Cyclosporin A (Cyclosporine A) is an immunosuppressant agent widely used in post-allogeneic organ transplant to reduce the activity of the immune system.
  2. glucocorticoid receptor agonist

    Dexamethasone is a potent synthetic member of the glucocorticoid class of steroid drugs. It acts as an anti-inflammatory and immunosuppressant. Its potency is about 20-30 times that of the naturally occurring hormone hydrocortisone and 4-5 times of prednisone.
  3. Integrin agonist

    Leukadherin 1 is an allosteric activator of CD11b/CD18. Increases CD11b/CD18-dependent cell adhesion to fibrinogen (EC50 = 4 μM).
  4. Rosmarinic acid is a phenylpropanoid isolated from Labiatae herbs.
  5. glucocorticoid receptor agonist

    Dexamethasone acetate (Dexamethasone 21-acetate) is a glucocorticoid receptor agonist.
  6. C5a receptor antagonist

    PMX-205 is a cyclic hexapeptide that acts as a potent antagonist of C5a receptor (C5aR; IC50 = 31 nM).
  7. glucocorticoid receptor agonist

    Dexamethasone Phosphate disodium is a is a water-soluble form of the synthetic glucocorticoid dexamethasone.
  8. glucocorticoid receptor agonist

    Dexamethasone palmitate (DXP) is a prodrug of Dexamethasone, which is a glucocorticoid receptor agonist. Dexamethasone palmitate (DXP) has a 47-fold lower affinity for the glucocorticoid receptor than Dexamethasone. Anti-inflammatory agent.
  9. Factor D inhibitor

    Danicopan (ACH-4471), a selective and orally active small-molecule factor D inhibitor, shows high binding affinity to human Factor D with Kd value of 0.54 nM.
  10. C5 inhibitor

    Complement C5-IN-1 (Compound 7) is a small-molecule inhibitor of complement component 5 protein (C5).
  11. Factor B inhibitor

    LNP023 is a highly potent factor B (FB) inhibitor with an IC50 value of 10 nM. LNP023 shows direct, reversible, and high-affinity binding to human FB (KD=7.9 nM).
  12. factor d inhibitor

    Complement factor D-IN-1 is a potent and selective small-molecule reversible factor d inhibitor, with IC50s of 0.006 and 0.05 μM in FD Thioesterolytic Fluorescent Assay and a MAC Deposition Assay, respectively.
  13. C3a receptor antagonist

    SB290157 trifluoroacetate is a potent and selective C3a receptor antagonist with an IC50 of 200 nM.
  14. Primates' C3 Inhibitor

    Compstatin, a 13-residue cyclic peptide, is a potent inhibitor of the complement system C3 with species specificity. Compstatin binds to baboon C3 and is resistant to proteolytic cleavage in baboon blood (similar to humans). Compstatin inhibits only the activation of primates' complement system. Compstatin exhibits IC50 values of 63 μM and 12 μM for classical and alterative complement pathway, respectively.

  15. AMY-101 TFA (Cp40 TFA), a peptidic inhibitor of the central complement component C3 (KD = 0.5 nM), inhibits naturally occurring periodontitis in non-human primates (NHPs). AMY-101 (Cp40) exhibits a favorable anti-inflammatory activity in models with COVID-19 severe pneumonia with systemic hyper inflammation.
  16. Complement factor C3 inhibitor

    POT-4 (AL-78898A), a Compstatin derivative, is a potent inhibitor of complement factor C3 activation. POT-4 can be used for age-related macular degeneration research
  17. C5aR1 antagonist

    JPE-1375 is a complement C5a receptor 1 (C5aR1) antagonist that effectively inhibits polymorphonuclear leukocyte mobilization with an EC₅₀ of 6.9 µM and reduces TNF levels with an EC₅₀ of 4.5 µM in mice. It is suitable for research in autoimmune and inflammatory diseases.
  18. NRP1 inhibitor

    EG01377 dihydrochloride is a potent, bioavailable, and selective inhibitor of neuropilin-1 (NRP1), with a Kd of 1.32 μM and IC₅₀ values of 609 nM for both NRP1-a1 and NRP1-b1 domains. It exhibits antiangiogenic, antimigratory, and antitumor activities.
  19. Perforin inhibitor

    Perforin-IN-2 (Compound 1) is a perforin inhibitor that blocks perforin-mediated cell lysis. It has potential to reduce graft rejection in the context of allogeneic bone marrow or stem cell transplantation.
  20. Lipoteichoic acid is an orally active compound with anti-inflammatory and antitumor properties. It is a key immune molecule found in Gram-positive bacteria that activates the complement system by upregulating C3 and inhibiting CD55. Lipoteichoic acid modulates macrophage autophagy via the PI3K/Akt/mTOR pathway, induces lung injury in mouse models, and inhibits melanin production.
  21. complement factor D inhibitor

    Vemircopan (ALXN2050) is an orally active inhibitor of complement factor D (FD). It is being investigated for the treatment of complement-mediated diseases, including myasthenia gravis, lupus nephritis, IgA nephropathy, and paroxysmal nocturnal hemoglobinuria (PNH).
  22. C5a receptor antagonist

    W-54011 is a potent, orally active, non-peptide antagonist of the C5a receptor. It inhibits the binding of \[¹²⁵I]-labeled C5a to human neutrophils with a Kᵢ of 2.2 nM. W-54011 also suppresses C5a-induced intracellular Ca²⁺ mobilization, chemotaxis, and reactive oxygen species (ROS) generation in human neutrophils, with IC₅₀ values of 3.1 nM, 2.7 nM, and 1.6 nM, respectively.
  23. C3a receptor agonist

    C3a Receptor Agonist 1 (a benzeneacetamide derivative) is a potent agonist of the C3a receptor. It holds potential for research in acute inflammation and complement system–mediated responses.
  24. Cemdisiran is an N-acetylgalactosamine (GalNAc)-conjugated small interfering RNA (siRNA) designed to suppress hepatic production of complement component 5 (C5). It is used in research focused on complement-mediated diseases.
  25. C5aR1 Agonist

    BM213 is a potent and selective agonist of C5aR1 that demonstrates antitumor activity in a mouse model of mammary carcinoma.
  26. C5a receptor agonist

    NDT 9513727 is a potent, selective, orally active, and competitive inverse agonist of the human C5a receptor (C5aR), with an IC₅₀ of 11.6 nM. It is suitable for research in human inflammatory diseases.
  27. Complement factor D inhibitor

    Pelecopan (BCX9930) is a potent, selective, and orally active inhibitor of complement factor D, with an IC₅₀ value of 14.3 nM. It targets factor D to prevent both intravascular and extravascular hemolysis in paroxysmal nocturnal hemoglobinuria (PNH) and is also applicable to other diseases mediated by the alternative complement pathway.
  28. C3aR ligand

    BR103 is a C3a receptor (C3aR)-specific small molecule ligand. It is useful for measuring ligand affinity to G protein-coupled receptors (GPCRs) through saturation and competitive binding assays.
  29. Tumor homing peptide

    LyP-1 is a cyclic 9-amino-acid tumor-homing peptide that selectively binds to p32 receptors, which are overexpressed on various tumor-associated cells.
  30. complement factor D (CFD) inhibitor

    Zoracopan is a selective inhibitor of complement factor D (CFD). Upon systemic administration (oral or intravenous), it preferentially accumulates in ocular tissues and is released in a sustained manner, primarily targeting the choroid–retinal pigment epithelium (C-RPE) and/or iridociliary body (I-CB).
  31. C5a receptor antagonist

    CP-447697 is a lipophilic C5a receptor antagonist with an IC₅₀ value of 31 nM. It is suitable for research related to inflammation and complement system modulation.
  32. Campneoside II is a natural compound isolated from the wood of *Paulownia tomentosa* var. *tomentosa*. It exhibits potent anti-complement activity.
  33. C1q/MBL inhibitor

    PIC1 PA is a 15-amino-acid peptide and a potent analog of PIC1 that inhibits classical pathway–mediated complement activation. It functions by disrupting the interaction between C1s-C1r-C1r-C1s/MASPs and the collagen-like region (CLR) of C1q or MBL. PIC1 PA specifically binds to the CLR of C1q, with a mean equilibrium dissociation constant (K\_D) of 33.3 nM for purified C1q.
  34. NRP-1 targeting peptide

    tLyP-1 peptide is a neuropilin-1 (NRP-1) targeting peptide with an IC₅₀ of 4 μM and the amino acid sequence CGNKRTR. It specifically binds to NRP-1, enabling selective targeting of tumor cells.
  35. C5a receptor antagonist

    L-156602 is a C5a receptor antagonist that inhibits inflammation and blocks the migration of monocytes and neutrophils to sites of infiltration in mouse inflammatory models. It also suppresses the efferent phase of delayed-type hypersensitivity (DTH).
  36. complement factor D inhibitor

    Tarvicopan is an inhibitor for complement factor D.
  37. Complement factor B inhibitor

    Lanoracopan is a potent inhibitor of complement factor B, with an IC₅₀ of 1.2 μM. It is suitable for research in inflammation- and immunity-related diseases.
  38. C5a receptor inhibitor

    CP-289,503 is an inhibitor of the complement C5a receptor, with an IC₅₀ of 1 μM. The C5a receptor mediates pro-inflammatory signaling by binding to C5a, a complement activation product that stimulates leukocyte and phagocyte activation, upregulates integrins, and induces degranulation of inflammatory cells, contributing to endothelial damage. By blocking C5a signaling, CP-289,503 may be useful in the treatment of various inflammatory diseases.
  39. CD11b agonist.

    ADH-503 ((Z)-Leukadherin-1 choline) is an orally active, allosteric agonist of CD11b, an integrin subunit expressed on myeloid cells. By activating CD11b, ADH-503 promotes the repolarization of tumor-associated macrophages (TAMs) toward a pro-inflammatory phenotype, reduces the infiltration of immunosuppressive myeloid cells into tumors, and enhances dendritic cell responses. These immunomodulatory effects support its potential use in cancer immunotherapy.
  40. complement C3a antagonist

    JR14a is a potent thiophene-based antagonist of the human complement C3a receptor (C3aR). It exhibits high selectivity for C3aR over the C5a receptor and effectively suppresses C3aR-mediated inflammatory responses. JR14a is a valuable tool for studying the role of C3aR in immune regulation and inflammation-related diseases.
  41. Phaseoloidin is a homogentisic acid glucoside isolated from *Nicotiana attenuata* trichomes. It plays a defensive role in the plant's resistance against lepidopteran herbivores by significantly reducing the growth of both specialist (*Manduca sexta*) and generalist (*Spodoptera littoralis*) larvae. In addition to its insect-deterrent properties, phaseoloidin also exhibits anti-complement activity, suggesting potential immunomodulatory applications.
  42. Complement Inhibitor

    Dextran sulfate sodium salt (DSS, MW 5000) primarily functions as a complement inhibitor, derived from a polymer of dehydrated glucose. This compound exhibits significant biological activities, including anticoagulation, antiviral properties, and anti-lipemic effects. Its mechanisms involve blocking HIV-1 adsorption to host cells, preventing natural killer (NK) cell-mediated cytotoxicity, and inhibiting instant blood-mediated inflammatory reactions (IBMIR). Dextran sulfate sodium salt (MW 5000) is valuable in research related to immunology, virology, and cardiovascular studies.
  43. Complement factor B synthesis reducer

    Agazisiran is a small interfering RNA (siRNA) that targets and reduces the synthesis of complement factor B, a key component in the alternative complement pathway. By modulating complement activation, Agazisiran has potential applications in research focused on autoimmune diseases, inflammatory conditions, and other disorders where complement dysregulation plays a critical role.
  44. Complement Inhibitor

    3-Phenoxybenzaldehyde is a compound identified as a complement inhibitor, exhibiting weak inhibition of the classical pathway of the complement system. This compound demonstrates hemolytic activity, making it relevant for studies related to immune response modulation and complement pathway research. Its applications extend to investigations in autoimmune diseases and therapies targeting complement-mediated cytotoxicity.
  45. C5 Complement Inhibitor

    Avacincaptad pegol is a pegylated RNA aptamer that selectively inhibits complement component C5. By preventing the cleavage of C5 into pro-inflammatory fragments C5a and C5b, this compound mitigates inflammatory cell recruitment and reduces retinal cell damage. Avacincaptad pegol has demonstrated potential in slowing the progression of geographic atrophy (GA) lesions and decreasing the risk of vision loss, making it a valuable tool for research involving age-related macular degeneration (AMD).
  46. Complement System Inhibitor

    Factor B-IN-1 is an inhibitor of Factor B, targeting the complement system to modulate its activity. This compound demonstrates significant inhibition of the alternative pathway of complement activation, making it a valuable tool for studying complement-mediated diseases and immune responses. Factor B-IN-1 is suitable for use in research applications focused on therapeutic interventions and the development of complement-targeted therapies.
  47. Anti-complement Activator

    Entadamide-A-β-D-glucopyranoside serves as an anti-complement activator, derived from the seeds of Entada phaseoloides. This compound exhibits significant biological activity by inhibiting complement activation, making it a valuable tool for research in immunology and inflammation. Its applications extend to studies focused on understanding the roles of the complement system in various diseases.
  48. Complement Factor D Inhibitor

    Complement Factor D-IN-2 is an inhibitor targeting complement factor D, effectively interrupting the complement cascade at a critical juncture within the alternative complement pathway. By inhibiting factor D, this compound presents valuable opportunities for investigating the role of complement activation in autoimmune diseases. Researchers can utilize Complement Factor D-IN-2 to explore therapeutic strategies aimed at modulating immune responses and mitigating the progression of complement-mediated pathologies.
  49. Human Complement C5

    Human complement C5 is a crucial component of the complement system, acting as a precursor to both the C5a anaphylatoxin and C5b, which play pivotal roles in immune responses. This protein is integral to the activation of the complement cascade, facilitating phagocytosis and the destruction of pathogens while enhancing adaptive immunity. Its study is essential for understanding immune system functions and developing therapeutic strategies against various diseases.
  50. Complement Factor D Inhibitor

    Refinicopan is a selective inhibitor of complement factor D, demonstrating an IC50 of 10 nM. This compound effectively inhibits rabbit erythrocyte hemolysis with an IC50 of 41 nM, indicating its capacity to modulate complement-mediated processes. Refinicopan's favorable pharmacokinetic and pharmacodynamic properties make it a valuable tool for investigating complement-related disorders and potential therapeutic interventions.

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