Complement System: Key Effector and Regulatory Mechanisms of the Immune System
Complement System: Key Effector and Regulatory Mechanisms of the Immune System
The complement system is one of the most important effector systems of humoral immunity and consists of a proteolytic enzyme cascade formed by multiple plasma proteins and membrane-bound proteins. As a core component of innate immunity, it is activated via the classical pathway, lectin pathway, and alternative pathway, and—through amplification—generates a variety of bioactive cleavage fragments and the membrane attack complex (MAC). These effectors play pivotal roles in pathogen elimination, immune complex handling, inflammation regulation, and modulation of adaptive immunity. Complement activity is tightly controlled by multiple layers of regulatory proteins to prevent excessive attack on host tissues; dysregulation of this system is closely associated with infectious diseases, autoimmune diseases, thrombotic microangiopathies, kidney diseases, and certain tumors and neurological disorders. With continued advances in basic complement research, diagnostic technologies, and targeted drug development, the complement system is emerging as a major target for precision prevention and treatment of immune-related diseases and for the development of novel biotherapeutics.
I. Overview of the Complement System
1.1 Basic Components and Nomenclature
The complement system consists of more than 30 soluble plasma proteins and membrane-associated proteins, including the classical components C1–C9 as well as regulatory factors such as factor B, factor D, factor H, factor I, and properdin, together with several complement receptors. Most complement proteins are synthesized by hepatocytes, while some are produced locally by monocyte–macrophages, epithelial cells, and others. In plasma, they typically circulate as inactive precursors and are activated only in response to infection, immune complexes, or tissue damage, through sequential proteolysis and assembly of multimolecular complexes. This activation produces enzymatically active C3 convertases, C5 convertases, and multiple effector fragments.
1.2 Features of Cascade Amplification and Fine Regulation
Complement activation is characterized by a “small trigger, large amplification” process. Initial activating signals are rapidly amplified through a series of enzymatic reactions, leading to massive generation of C3b, C5a, and MAC, which together produce strong immune effector functions. In parallel, the system is constrained by multiple soluble and membrane-bound regulatory proteins that impose rate-limiting and negative feedback control at key steps, thereby preventing uncontrolled activation and host tissue injury. The dynamic balance between high amplification potential and refined regulation is critical for complement to fulfill its physiological functions without becoming pathogenic.
1.3 Relationship with Innate and Adaptive Immunity
The complement system is a major component of innate immunity, capable of direct activation via recognition of pathogen surface structures, low-level spontaneous hydrolysis, and pattern-recognition molecules. At the same time, through the classical pathway and mechanisms such as C3d–CR2-mediated B-cell co-stimulation, complement is tightly integrated with adaptive immunity: it amplifies antibody effector functions and modulates the magnitude and quality of antibody responses. Complement is therefore regarded as a central bridge linking innate and adaptive immunity.
II. Components and Classification of the Complement System
2.1 Core Components of the Cascade
The core complement components involved in cascade activation include the C1 complex (C1q, C1r, C1s), C2–C9, factor B, factor D, properdin, as well as pattern-recognition molecules such as mannose-binding lectin (MBL) and ficolins. In the resting state, these components typically exist as zymogens or non-covalent complexes. Upon activation, specific proteolytic events generate cleavage fragments that assemble into C3 convertases, C5 convertases, and MAC, which directly mediate pathogen lysis, opsonization, and amplification of inflammation.
2.2 Regulatory Proteins
Regulatory proteins constrain the intensity and spatial range of complement activation and prevent excessive attack on host cells. Major fluid-phase regulators include C1 inhibitor (C1INH), factor H, factor I, and others, which limit activation of the classical, lectin, and alternative pathways by inhibiting key enzymatic steps or promoting C3b degradation. Membrane-bound regulators include decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46), complement receptor 1 (CR1, CD35), homologous restriction factors, and MAC-inhibitory protein CD59. These molecules accelerate dissociation of convertases or block MAC assembly, providing “self-protection labels” for host cells.
2.3 Complement Receptors
Complement receptors are expressed on a wide variety of immune and some non-immune cells and recognize complement fragments to transmit downstream signals. Representative receptors include CR1, which recognizes C3b/C4b; CR2, which recognizes C3d/C3dg; CR3 and CR4, which recognize iC3b; and C5aR, which recognizes C5a. These receptors participate in cell adhesion, opsonophagocytosis, leukocyte chemotaxis, B-cell co-stimulation, and immune memory formation, thereby functionally linking complement to cellular immunity.
III. Complement Activation Pathways and the Terminal Pathway
3.1 Classical Pathway
(1) Initiation and the C1 Complex
The classical pathway is primarily triggered by antigen–antibody complexes and is the major complement activation route during adaptive immune responses. Upon binding antigen, IgM or IgG1–3 undergo conformational changes exposing C1q-binding sites in the Fc region. C1q binding to clustered Fc domains induces autoactivation of C1r, which in turn activates C1s. The resulting C1 complex acquires proteolytic activity and initiates the cascade.
(2) Formation of C3 and C5 Convertases
Activated C1s sequentially cleaves C4 and C2. C4b covalently attaches to target surfaces via its reactive thioester bond and associates with C2a to form C4b2a, the C3 convertase of the classical pathway. C4b2a cleaves large amounts of C3 into C3a and C3b. C3b deposits on target surfaces to mediate opsonization, and some C3b associates with C4b2a to form C4b2a3b, the C5 convertase of the classical pathway, which cleaves C5 and feeds into the terminal pathway.
3.2 Lectin Pathway
(1) Pattern-Recognition Molecules and MASP-Mediated Activation
The lectin pathway, belonging to innate immunity, is initiated when pattern-recognition molecules such as MBL and ficolins recognize and bind specific carbohydrate structures (e.g., mannose, N-acetylglucosamine) on microbial surfaces. Ligand-bound MBL or ficolin recruits and activates MBL-associated serine proteases MASP-1 and MASP-2. Activated MASP-2 exhibits C1s-like activity and cleaves C4 and C2 to generate C4b2a, identical to the C3 convertase of the classical pathway.
(2) Convergence with the Classical Pathway Downstream
From the stage of C3 convertase onward, the lectin and classical pathways share the same amplification mechanisms and terminal effector route. C4b2a cleaves C3 to generate C3b and C3a; C3b deposition and participation in C5 convertase assembly lead both pathways to converge completely at the levels of C3 and C5, culminating in MAC formation and execution of lytic and inflammatory effects.
3.3 Alternative Pathway
(1) Spontaneous C3 Hydrolysis and Initial C3 Convertase
The alternative pathway does not require antibodies or MBL and represents the most primitive “continuous surveillance” mode of complement activation. In plasma, C3 undergoes low-level spontaneous hydrolysis to form C3(H₂O), which can bind factor B and, upon cleavage by factor D, generate C3(H₂O)Bb, a fluid-phase C3 convertase. This enzyme cleaves C3 to generate C3b and provides the basis for alternative pathway initiation on appropriate surfaces.
(2) C3bBb Amplification Loop and C5 Convertase Formation
When C3b deposits on “non-self” surfaces such as pathogens or damaged cells and is not rapidly inactivated by factor H and factor I, it can bind factor B and, following cleavage by factor D, form C3bBb, the C3 convertase of the alternative pathway. Properdin stabilizes C3bBb, markedly prolonging its half-life and establishing a powerful positive feedback loop that cleaves more C3 to generate additional C3b. Association of an additional C3b yields C3bBb3b, the C5 convertase of the alternative pathway, which cleaves C5 and drives the terminal pathway.
3.4 Terminal Common Pathway and Membrane Attack Complex
(1) C5 Cleavage and Assembly of C5b-9
Regardless of whether activation is initiated via the classical, lectin, or alternative pathway, all three converge at the level of C5. C5 convertases cleave C5 into the small fragment C5a and the larger fragment C5b. C5b sequentially recruits C6, C7, and C8 to form C5b-8, which then binds multiple C9 molecules to assemble the C5b-9 MAC on target membranes, forming a transmembrane pore structure.
(2) MAC-Mediated Membrane Damage and Cell Lysis
Insertion of MAC into the membrane markedly alters membrane permeability, causing abnormal ion and water fluxes and ultimately leading to osmotic lysis or functional impairment of target cells. Gram-negative bacteria, some enveloped viruses, and certain damaged host cells are particularly susceptible to MAC, making this one of the most characteristic mechanisms of direct complement-mediated cytotoxicity.
IV. Major Biological Functions of the Complement System
4.1 Cytolytic Effects and Pathogen Elimination
Through the terminal pathway, MAC directly disrupts the membranes of complement-sensitive microorganisms and damaged cells, resulting in osmotic imbalance and lysis. This direct killing mechanism is an important component of innate defense. For some Gram-negative bacteria and Neisseria species, complement-mediated bacteriolysis is especially critical for infection control.
4.2 Opsonization and Enhanced Phagocytosis
C3b, C4b, and their degradation product iC3b deposited on pathogens or immune complexes act as key opsonins. These are recognized by receptors such as CR1, CR3, and CR4 on neutrophils and monocyte–macrophages, markedly increasing phagocytic efficiency. This opsonization function positions complement as an essential link between humoral effectors and phagocytic clearance and facilitates rapid removal of pathogens and immune complexes.
4.3 Chemotaxis, Inflammatory Amplification, and Vascular Responses
C3a, C4a, and C5a are collectively known as anaphylatoxins, among which C5a is the most bioactive. By binding to C5aR, C5a strongly chemoattracts neutrophils, monocytes, and other inflammatory cells to sites of injury or infection and promotes their degranulation and activation. In parallel, it induces vasodilation and increased vascular permeability, synergizing with other mediators to amplify local inflammation. While beneficial for rapid recruitment of immune cells and pathogen clearance, uncontrolled complement activation can cause tissue damage and may contribute to systemic inflammatory response syndrome.
4.4 Immune Complex Clearance and Modulation of Adaptive Immunity
When complement deposits on circulating immune complexes, these complexes can be recognized and bound by CR1 on erythrocytes and subsequently transferred to macrophages in the liver and spleen for clearance. This process reduces immune complex deposition in glomeruli and vessel walls and lowers the risk of tissue injury. In adaptive immunity, C3d bound to antigen engages CR2 (CD21) on B cells, co-stimulating B-cell receptor (BCR) signaling, lowering the threshold for B-cell activation, and promoting antibody responses and memory B-cell formation. Complement thereby plays an important role in quality control of antibody responses.
V. Complement Regulatory Proteins and Self-Protection Mechanisms
5.1 Fluid-Phase Regulatory Factors
(1) C1INH and Restriction of Classical/Lectin Pathways
C1 inhibitor (C1INH) forms complexes with C1r/C1s and MASPs, inhibiting their serine protease activities and thus limiting overactivation of the classical and lectin pathways. C1INH deficiency leads to excessive activation of complement and the kallikrein–kinin system, forming the key pathogenic basis of hereditary angioedema.
(2) Factor H, Factor I, and C3b Regulation
Factor H preferentially recognizes host surfaces bearing self-associated markers such as sialylated glycans, binds C3b, and facilitates dissociation of factor B, thereby inhibiting alternative pathway amplification on host cells. Factor I, in the presence of cofactors such as factor H, CR1, or MCP, cleaves C3b to iC3b and further fragments, terminating its convertase activity. This collaborative regulatory axis is central to restraining basal alternative pathway activation and preventing complement attack on host structures.
5.2 Membrane-Bound Regulatory Proteins
(1) DAF, MCP, and Dissociation of Convertases
DAF (CD55) accelerates the decay of C3 and C5 convertases (e.g., C4b2a, C3bBb), shortening their half-lives and suppressing complement amplification. MCP (CD46) acts as a membrane-bound cofactor for factor I, accelerating inactivation of C3b and C4b on cell surfaces. Together, these regulators help host cells maintain low levels of complement deposition even in complement-activating environments.
(2) CD59 and Inhibition of MAC Formation
CD59 (protectin) binds to C8 and C9 during terminal complex assembly and prevents C9 polymerization and insertion, thereby blocking MAC formation on host cell membranes. Loss of CD59 function or reduced expression renders erythrocytes, platelets, and endothelial cells more susceptible to complement-mediated lysis and damage and contributes to certain hemolytic and thrombotic conditions.
5.3 Overall Balance of Positive and Negative Regulation
Fluid-phase and membrane-bound regulators together form a multilayered protective system that prevents persistent complement amplification on host cells while allowing efficient activation on pathogen surfaces lacking self-markers. Properdin, which stabilizes alternative pathway convertases, functions as a positive regulator, whereas factor H, factor I, and other inhibitors exert negative control. The balance between these opposing forces determines the overall “activation–inhibition” status of the complement system; disruption of this balance often leads to complement-associated diseases.
VI. Complement System and Disease
6.1 Congenital Deficiencies and Susceptibility to Infection
Congenital deficiencies of complement components significantly increase susceptibility to specific infections. Deficiencies of C1q, C2, or C4 often cause severe impairment of immune complex clearance and are associated with systemic lupus erythematosus (SLE)-like manifestations. C3 deficiency leads to broad susceptibility to bacterial pathogens. Deficiencies in terminal components C5–C9 predispose to recurrent Neisseria infections. Defects in alternative pathway components or regulators can also increase susceptibility to pyogenic bacteria or enveloped viruses, underscoring the indispensable role of complement in innate immune defense.
6.2 Autoimmune and Immune Complex–Mediated Diseases
In SLE, rheumatoid arthritis, and certain immune complex–mediated glomerulonephritides, persistent immune complex formation drives chronic activation of the classical pathway, resulting in reduced serum levels of C3 and C4 and prominent complement deposition in tissues. Complement activation products contribute to tissue injury through inflammatory amplification, endothelial damage, and neutrophil activation, and represent important drivers of disease activity and severity. Complement levels and activation markers are commonly used as indicators for disease activity assessment and prognosis.
6.3 Complement-Mediated Hemolytic and Thrombotic Disorders
In paroxysmal nocturnal hemoglobinuria (PNH), acquired mutations in hematopoietic stem cells cause loss of GPI-anchored proteins, leading to deficiency of CD55 and CD59 on erythrocytes and resulting in heightened sensitivity to complement-mediated lysis. Clinically this manifests as chronic intravascular hemolysis and a high risk of venous thrombosis. In atypical hemolytic uremic syndrome (aHUS), mutations in factor H, factor I, or MCP result in uncontrolled alternative pathway activation, triggering endothelial damage, platelet activation, and microvascular thrombosis and presenting with hemolytic anemia, thrombocytopenia, and acute kidney injury.
6.4 Tumors, Ischemia–Reperfusion Injury, and Neurological Diseases
In the tumor microenvironment, complement exhibits dual roles. On one hand, antibody-dependent complement-mediated cytotoxicity contributes to tumor immune surveillance; on the other hand, chronic low-level complement activation and C5a–C5aR signaling may promote recruitment of immunosuppressive cells and tumor angiogenesis, facilitating tumor progression. In ischemia–reperfusion injury, complement activation products exacerbate endothelial damage and inflammation and participate in organ injury such as myocardial infarction and stroke. In the nervous system, complement is implicated in synaptic pruning, microglial activation, and neuroinflammation and is increasingly recognized as a contributor to the pathogenesis of certain neurodegenerative diseases.
VII. Complement Detection and Experimental Applications
7.1 Serum Complement Levels and Functional Assessment
(1) Quantification of C3, C4, and Related Components
In clinical and research settings, serum levels of C3, C4, C1q, MBL, and sC5b-9 are commonly quantified by immunoturbidimetry or ELISA to assess complement consumption and activation. In diseases such as SLE, persistently reduced C3 and C4 levels often indicate ongoing disease activity or increased immune complex burden.
(2) CH50 and AH50 Hemolytic Activity Assays
Total classical pathway hemolytic activity (CH50) is evaluated by measuring the ability of complement-containing serum to lyse antibody-sensitized sheep erythrocytes and reflects the combined function of classical pathway components. Alternative pathway hemolytic activity (AH50) is assessed under buffer conditions that inhibit the classical and lectin pathways, using specific erythrocytes as targets to evaluate alternative pathway function. These assays help screen for congenital complement deficiencies and assess complement dysfunction.
7.2 Complement Detection at Tissue and Cellular Levels
(1) Western Blot and Fragment Analysis
Western blotting can be used to detect complement fragments (e.g., C3b, iC3b, C3d, C5b-9) in plasma, cell culture supernatants, or tissues and to analyze the extent and pathway specificity of complement activation. Use of fragment-specific antibodies helps distinguish upstream component deficiency, regulator abnormalities, or excessive activation of the terminal pathway.
(2) Immunofluorescence/Immunohistochemistry and Complement Deposition
In kidney biopsies, skin, and other tissues, immunofluorescence or immunohistochemistry is used to assess deposition patterns of C3, C1q, C4d, and C5b-9, aiding in disease classification and mechanistic interpretation. For example, “C3-dominant” glomerular deposition suggests alternative pathway–driven complement-mediated glomerulopathy, whereas co-deposition of C1q, C4, and immunoglobulins supports immune complex–mediated nephritis.
7.3 In Vitro Functional Assays and Model Systems
(1) Complement-Dependent Cytotoxicity and Bacteriolysis Assays
In complement-dependent cytotoxicity (CDC) assays, target cells expressing specific antigens are incubated with corresponding antibodies and complement-active serum to evaluate complement-dependent cell lysis. Similarly, bacteriolysis assays measure bacterial sensitivity to complement and assess how complement deficiencies or inhibitors affect bactericidal capacity.
(2) Complement Inhibitors and Reconstituted Complement Systems
By adding specific complement inhibitors (e.g., anti-C5 antibodies, small-molecule inhibitors) or using heat-inactivated serum in combination with defined purified complement components, in vitro systems can be tailored to dissect the roles of individual components and pathways in cellular inflammatory responses, phagocytosis, adhesion, and signal transduction. Such experimental designs are powerful tools for screening complement-targeted drugs and elucidating their mechanisms of action.
VIII. Complement-Based Diagnostic and Therapeutic Strategies
8.1 Complement-Related Diagnostic Biomarkers
Complement components and activation products serve as important clinical and research biomarkers. Serum C3, C4, CH50, and AH50 are used to assess overall complement function and consumption, while activation products such as C3a, C5a, and sC5b-9 indicate acute activation status. Tissue deposition patterns of C1q, C3, C4d, and C5b-9 help classify the pathology and predict prognosis in nephritis, transplant rejection, and vasculitis.
8.2 Complement-Targeted Therapeutics and Inhibitors
Targeted inhibitors directed at key complement nodes have entered clinical use. Anti-C5 monoclonal antibodies block C5 cleavage and MAC formation and are used to treat PNH, aHUS, and other complement-mediated diseases. C1INH preparations are used for acute attack control and prophylaxis in hereditary angioedema. Next-generation agents targeting C3, factor B, factor D, or C5aR—developed as small molecules or antibodies—are undergoing clinical trials in multiple diseases and offer new options for hemolytic disorders, complement-mediated nephropathies, and selected autoimmune diseases.
8.3 Complement Fragments as Vaccine Adjuvants and Emerging Applications
Complement fragments, especially C3d, have been explored as vaccine adjuvants: by binding CR2 on B cells, they enhance BCR signaling and markedly improve the magnitude and durability of vaccine-induced antibody responses. In addition, diagnostic reagents and imaging probes based on complement receptors and fragments are being developed to enable precise localization and monitoring of inflammatory foci, tumor microenvironments, and ischemia–reperfusion injury.
IX. Related Aladdin Products
Catalog No. | Product Name | Category | Source | Recommended Application | Remarks |
Complement C5, human plasma | Native protein | Human plasma–derived complement C5 | Studies of terminal complement pathway function, C5 cleavage, and hemolytic models | Can be used together with anti-C5 antibodies or recombinant C5-related proteins in in vitro inhibition assays | |
Complement C4c, human plasma | Native protein | Human plasma–derived complement C4c | Studies of classical pathway activation, C4 fragment detection, and related mechanisms | Suitable for projects on classical pathway–related complement activation and immune complex biology | |
Complement C3c, human plasma | Native protein | Human plasma–derived complement C3c | Research on cleavage, deposition, and regulation of central component C3 | Applicable to assessment of complement activation and C3-associated signaling pathways | |
C1r enzyme from normal human serum | Native protein | C1r from normal human serum | Studies of early classical pathway activation and C1 complex function | Suitable for research on C1 complex composition, activation mechanisms, and inhibitor screening | |
C1q protein from normal human serum | Native protein | C1q from normal human serum | Studies of classical pathway initiation, C1q ligand recognition, and apoptotic cell clearance | Applicable to autoimmune disease–related research and clearance of apoptotic cells and immune complexes | |
C1 esterase inhibitor, human plasma | Native protein | Human plasma–derived C1 inhibitor | Research on C1 inhibition, vascular permeability, and complement regulation mechanisms | Suitable for models of complement overactivation and studies of regulatory pathways | |
Vilobelimab (anti-Complement C5) | Antibody | Target: complement C5 | C5 blockade and studies of terminal complement pathway inhibition | Suitable for in vitro complement inhibition models and antibody–antigen interaction studies | |
Tesidolumab (anti-Complement C5) | Antibody | Target: complement C5 | Studies of terminal pathway regulation and C5a/C5b generation | Applicable to combined evaluation and comparative studies of complement inhibitors | |
Ravulizumab (anti-Complement C5) | Antibody | Target: complement C5 | C5-specific blockade and long-term inhibition strategies | Suitable for in vitro validation and PK/PD-oriented studies in complement-related disease models | |
Crovalimab (anti-Complement C5) | Antibody | Target: complement C5 | C5 inhibition and evaluation of antibody affinity and functional activity | Can be used alongside other anti-C5 antibodies for cross-comparison and synergy studies | |
Pozelimab (anti-Complement C5) | Antibody | Target: complement C5 | Inhibition of C5-dependent hemolysis and studies of the terminal complement pathway | Suitable for in vitro hemolysis assays and monitoring MAC formation | |
Eculizumab (anti-Complement C5) | Antibody | Target: complement C5 | C5 inhibition, hemolysis models, and terminal pathway blockade | Commonly used as a reference anti-C5 antibody in comparative experiments | |
Lampalizumab (anti-Complement Factor D) | Antibody | Target: complement factor D | Studies of alternative pathway activation control and factor D–related mechanisms | Suitable for evaluation of selective inhibition strategies targeting the alternative pathway | |
ANX005 (anti-C1q) | Antibody | Target: C1q | Inhibition of the classical pathway and studies of C1q-mediated clearance and inflammation | Can be used together with C1q protein for classical pathway–specific research | |
Gefurulimab (anti-C5 & Albumin) | Antibody | Targets: C5 and albumin | Studies of C5 inhibition and plasma protein binding properties | Suitable for validating long-acting complement inhibition strategies and albumin-fusion concepts | |
Avdoralimab (anti-C5AR1) | Antibody | Target: C5a receptor 1 (C5aR1) | Studies of C5a signaling, receptor blockade, and inflammatory pathways | Applicable to chemotaxis, cytokine release, and innate immune response assays | |
Lendalizumab (anti-Complement C5) | Antibody | Target: complement C5 | Inhibition of the terminal pathway and C5-directed blockade | Suitable for functional comparison among different anti-C5 antibodies | |
ARGX-117 (anti-Complement C2) | Antibody | Target: complement C2 | Studies of the classical/lectin pathway branch point and C2-related regulation | Suitable for validation of upstream complement regulatory targets and mechanistic research | |
NGM621 (anti-Complement C3) | Antibody | Target: complement C3 | Blockade of central component C3 and global modulation of the complement cascade | Applicable to in vitro evaluation of strategies that simultaneously inhibit multiple pathways | |
Recombinant CD21 Antibody | Antibody | Target: CD21 | Studies of the B-cell receptor complex and complement receptor signaling | Suitable for research on B-cell activation, antigen presentation, and complement receptor function | |
Recombinant C4d Antibody | Antibody | Target: C4d | Detection of C4d deposition and complement activation markers | Suitable for transplant immunology and C4d staining in tissue sections | |
Recombinant C3 Antibody | Antibody | Target: C3 | Detection of C3/C3b deposition and assessment of complement activation | Applicable to WB, IHC, or immunofluorescence for complement deposition | |
Recombinant Factor H Antibody | Antibody | Target: factor H | Studies of factor H function and regulation as a complement regulator | Suitable for research on negative regulation of the alternative pathway and genetic variants | |
Recombinant CD35 Antibody | Antibody | Target: CD35 (CR1) | Studies of CR1 expression and function | Applicable to detection of cell-surface CR1 and complement-binding analyses | |
Recombinant CD35 Antibody | Antibody | Target: CD35 (CR1) | Studies of CR1-mediated immune complex clearance and regulation | Can be used together with Ab094648 for cross-validation of signals and specificity | |
Recombinant C1QB Antibody | Antibody | Target: C1QB | Studies of C1q subunit structure and function | Suitable for research on C1 complex composition and ligand-recognition mechanisms | |
Recombinant Factor D/CFD Antibody | Antibody | Target: factor D | Studies of alternative pathway activation and factor D regulation | Can be used in combination with Lampalizumab and other factor D–related antibodies | |
Recombinant C9 Antibody | Antibody | Target: C9 | Studies of MAC formation and C9 deposition | Suitable for detection and visualization of MAC-related pathways and cell lysis | |
CD21 Mouse mAb | Antibody | Mouse monoclonal antibody targeting CD21 | Identification of B-cell subsets and CD21-related signaling | Applicable to flow cytometry and immunostaining | |
C1s Mouse mAb | Antibody | Mouse monoclonal antibody targeting C1s | Studies of C1s expression and activity in the classical pathway | Suitable for C1 complex functional assays and inhibitor screening | |
GC1q R Mouse mAb | Antibody | Mouse monoclonal antibody targeting GC1qR | Studies of C1q receptor–related signaling and cell-surface receptors | Applicable to research on C1q-mediated cellular responses and receptor localization | |
Complement C3 Antibody | Antibody | Target: complement C3 | Detection of C3 expression and deposition in tissues/cells | Commonly used for IHC/IF assessment of complement deposition and inflammation | |
C1QL3/CTRP13 Antibody | Antibody | Target: C1QL3/CTRP13 | Studies of C1q-related family proteins in metabolism and neural function | Suitable for exploring roles of C1q-related ligands in the CNS and metabolic regulation | |
Factor B Antibody | Antibody | Target: complement factor B | Studies of alternative pathway activation and factor B function | Applicable to monitoring alternative pathway activation and mechanistic studies | |
Complement C4 Antibody | Antibody | Target: complement C4 | Detection of classical pathway activation and C4 deposition | Suitable for IHC or immunofluorescence analysis of C4 deposition | |
Recombinant Human Complement Component C3a Protein | Recombinant protein | Human recombinant C3a | Studies of C3a-mediated inflammation, chemotaxis, and receptor signaling | Can be used as a C3a receptor agonist/ligand in in vitro functional assays | |
Recombinant Human Complement Factor B Protein | Recombinant protein | Human recombinant factor B | Studies of alternative pathway formation and C3 convertase assembly | Suitable for reconstituting in vitro alternative pathway systems and enzymatic activity analyses | |
Recombinant Human Complement C1s Protein | Recombinant protein | Human recombinant C1s | Studies of C1 complex activation and substrate cleavage in the classical pathway | Can be combined with C1q/C1r to investigate full C1 complex function | |
Recombinant Human Factor I/CFI Protein | Recombinant protein | Human recombinant factor I | Studies of C3b/C4b cleavage and mechanisms of complement negative regulation | Suitable for validating cooperative effects of factor H, CR1, and other regulatory proteins | |
Recombinant Human Serpin G1 Protein | Recombinant protein | Human Serpin G1 (C1 inhibitor) | Studies of C1 inhibition, vascular permeability, and inflammation control | Can be compared with native C1 inhibitor in mechanistic and structure–function analyses | |
Recombinant Human CD59 Protein | Recombinant protein | Human recombinant CD59 | Studies of MAC inhibition and cell protection | Suitable for models of MAC formation blockade and protection from complement-mediated lysis | |
Recombinant Human Complement C7 Protein | Recombinant protein | Human recombinant C7 | Studies of MAC assembly and C5b-9 complex formation | Applicable to in vitro reconstruction of the terminal attack complex | |
Recombinant Human Complement Component C2 Protein | Recombinant protein | Human recombinant C2 | Studies of C3 convertase formation and classical/lectin pathway activation | Suitable for mechanistic studies of upstream complement activation and convertase assembly | |
Recombinant Human Complement Component C1r Protein | Recombinant protein | Human recombinant C1r | Studies of C1 complex composition and activation mechanisms | Can be combined with C1q/C1s to construct in vitro C1 activation models | |
Recombinant Human Complement Component C9 Protein | Recombinant protein | Human recombinant C9 | Studies of MAC formation and the terminal complement pathway | Suitable for research on C5b-9 complex structure and pore-forming mechanisms | |
Recombinant Human CD21 Protein | Recombinant protein | Human recombinant CD21 | Studies of complement receptor–ligand interactions and B-cell signaling | Suitable for receptor-binding assays, surface coating, and in vitro binding experiments | |
Recombinant Human CD21 Protein | Recombinant protein | Human recombinant CD21 | Studies of CD21-mediated complement-dependent recognition and signal amplification | Can be used together with CD21 antibodies to analyze receptor function and ligand interactions | |
Recombinant Human C1qR1/CD93 Protein | Recombinant protein | Human recombinant C1qR1/CD93 | Studies of C1q receptor–related clearance, adhesion, and signaling | Suitable for receptor functional studies in macrophage and endothelial cell models |
As a key effector module of innate immunity and a crucial bridge between innate and adaptive immune responses, the complement system plays indispensable roles in pathogen clearance, inflammation regulation, immune complex handling, and maintenance of tissue homeostasis. Its highly amplified enzymatic cascade and multilayered regulatory network together form a refined and efficient defense system, but disruption of this balance can give rise to a spectrum of pathological states, including infection susceptibility, autoimmune diseases, hemolytic and thrombotic disorders, renal injury, and even malignancies and neurological diseases. With deeper insights into complement biology, structure–function relationships, and molecular regulatory mechanisms—and in combination with high-throughput omics and precision medicine approaches—diagnostic biomarkers and targeted therapies directed at distinct complement pathways and critical nodes are emerging at an accelerating pace.
Aladdin: https://www.aladdinsci.com/