Inflammatory Mediators — Complement, Cytokines, Arachidonic Acid

COMPLEMENT SYSTEM

• Classical Pathway Activation: Initiated by IgG or IgM antibody binding to antigen-antibody complexes, triggering C1 complex (C1q, C1r, C1s) binding to the Fc region. Sequential proteolytic cleavage produces C4a and C2a fragments and assembles C3 convertase (C4b2a). This is the primary pathway in immune complex-mediated diseases and autoimmune conditions.

• Alternative Pathway Activation: Constitutive low-level C3 hydrolysis generates C3i, which binds Factor B and forms C3 convertase without antibody requirement. Amplification loop involves Factor D proteolysis and properdin stabilization. Activated by microbial polysaccharides, lipopolysaccharides (LPS), and fungal cell walls—explaining rapid innate response to pathogens.

• Lectin Pathway Activation: Mannose-binding lectin (MBL) and ficolins recognize pathogen-associated molecular patterns (PAMPs) on bacteria and fungi; similar to C1-mediated initiation but bypasses IgG/IgM requirement. Critical in neonates and IgA nephropathy.

• C3 Cleavage and Amplification: All three pathways converge at C3 convertase generation. C3a (small fragment) acts as potent anaphylatoxin binding C3aR receptors on mast cells, neutrophils, and endothelium, triggering degranulation, vasodilation, and increased vascular permeability. C3b (large fragment) covalently deposits on pathogen/cell surfaces, opsonizing them for phagocytosis via complement receptors (CR1, CR3, CR4). Positive feedback amplification multiplies C3 cleavage 100-fold.

• C5 Cleavage and MAC Formation: C5 convertase (C4b2a3b or C3bBb3b) cleaves C5 into C5a and C5b. C5a is the most potent chemotactic and anaphylatoxic mediator (100× more potent than C3a), recruiting neutrophils and macrophages via C5aR and atypical chemokine receptor 1 (ACKR1). C5b initiates membrane attack complex (MAC) assembly with C6, C7, C8, and C9, forming transmembrane pores causing osmotic lysis of bacteria and cells.

• Regulation and Complement-Mediated Tissue Damage: C1-inhibitor (C1-INH), Factor H, Factor I, and membrane regulatory proteins (CD55/DAF, CD46/MCP, CD59) prevent excessive activation. Deficiencies → hereditary angioedema (C1-INH deficiency) and hemolytic uremic syndrome (Factor H mutations). Uncontrolled complement deposition causes membranoproliferative glomerulonephritis, post-infectious glomerulonephritis, and vasculitis.

CYTOKINES: PRO-INFLAMMATORY AXIS

• Tumor Necrosis Factor-α (TNF-α): Produced primarily by activated macrophages, dendritic cells, and T cells in response to LPS, IFN-γ, and other PAMPs. Binds TNFR1 (ubiquitous) and TNFR2 (immune cells), activating both NF-κB inflammatory signaling (pro-inflammatory) and caspase-8 apoptotic pathways. Systemic TNF-α causes fever (hypothalamic thermoregulation), septic shock (profound vasodilation and hypotension), and cachexia via metabolic reprogramming. Local TNF-α recruits neutrophils via upregulation of adhesion molecules (E-selectin, ICAM-1) on endothelium and induces IL-1 and IL-6 production, creating inflammatory amplification cascade.

• Interleukin-1 (IL-1α and IL-1β): IL-1β (principal secreted form) produced by macrophages, endothelial cells, and fibroblasts. Processed from pro-IL-1β by inflammasome (NLRP3 complex)-activated caspase-1. Binds IL-1R on T cells, endothelium, and hypothalamus, inducing fever, IL-6 production, vasodilation, and enhanced leukocyte adhesion. Synergizes with TNF-α in sepsis pathogenesis; therapeutic IL-1 blockade (anakinra, canakinumab) effective in autoinflammatory syndromes and acute gout.

• Interleukin-6 (IL-6): Key acute phase response mediator; produced by macrophages and endothelial cells stimulated by TNF-α and IL-1. Stimulates hepatic synthesis of acute phase proteins (CRP, serum amyloid A, fibrinogen) and hypothalamic fever. Elevated in infection, trauma, malignancy, and chronic inflammation. IL-6 blockade (tocilizumab, sarilumab) effective in rheumatoid arthritis and cytokine release syndrome.

• Interleukin-8 (IL-8/CXCL8): Primary chemokine for neutrophil recruitment; produced by macrophages, endothelial cells, and fibroblasts in response to TNF-α and IL-1. Binds CXCR1 and CXCR2 on neutrophils, triggering directional migration (chemotaxis) and degranulation. Dramatically elevated in acute bacterial infection, acute respiratory distress syndrome (ARDS), and acute pancreatitis. Excess IL-8 contributes to neutrophil-mediated tissue damage in cystic fibrosis and chronic obstructive pulmonary disease (COPD).

• Interleukin-12 (IL-12) and Interferon-γ (IFN-γ): IL-12 produced by dendritic cells and macrophages; promotes Th1 differentiation and IFN-γ production by T cells and NK cells. IFN-γ amplifies macrophage activation (upregulates TNF-α, IL-1, antigen presentation), enhances microbicidal function, and promotes immunoglobulin class switching to IgG. Excessive IFN-γ contributes to granulomatous inflammation and tissue destruction in tuberculosis and sarcoidosis.

CYTOKINES: ANTI-INFLAMMATORY AND REGULATORY AXIS

• Interleukin-10 (IL-10): Produced by macrophages, regulatory T cells (Tregs), and some B cells; suppresses TNF-α, IL-6, IL-8, and IL-12 production via STAT3 signaling. Decreases MHC expression and co-stimulatory molecule upregulation, dampening adaptive immunity. Deficiency or dysfunction predisposes to inflammatory bowel disease and celiac disease. IL-10 therapy under investigation for sepsis immunosuppression.

• Transforming Growth Factor-β (TGF-β): Produced by macrophages, fibroblasts, and Tregs in late/chronic inflammation; suppresses Th1/Th17 responses, inhibits inflammasome activation, and promotes regulatory T cell (Foxp3+) differentiation. Shifts macrophages toward M2 anti-inflammatory phenotype, promotes tissue remodeling and fibrosis (excessive TGF-β → pulmonary fibrosis, hepatic cirrhosis, systemic sclerosis). TGF-β receptor antagonists in development for fibrotic diseases.

ARACHIDONIC ACID METABOLITES

• Cyclooxygenase (COX) Pathway: Phospholipase C activation (by TNF-α, IL-1, complement) liberates arachidonic acid from membrane phospholipids. COX-1 (constitutive) and COX-2 (inducible by inflammatory stimuli) catalyze conversion to prostaglandin H2 (PGH2), substrate for cell-type-specific synthases. Prostaglandin E2 (PGE2): potent vasodilator and fever inducer (acts on EP3 hypothalamic receptors); also enhances vascular permeability and pain perception. Prostaglandin I2 (prostacyclin): endothelial-derived vasodilator and antiplatelet agent (platelet adenylyl cyclase inhibition). Thromboxane A2 (TXA2): platelet-derived vasoconstrictor and platelet aggregator. NSAIDs inhibit COX, reducing prostaglandin synthesis, providing analgesia, anti-inflammation, and antipyresis. Aspirin irreversibly acetylates COX, uniquely blocking TXA2 (explaining antiplatelet effect).

• Lipoxygenase (LOX) Pathway: 5-Lipoxygenase (5-LOX) converts arachidonic acid to leukotriene A4 (LTA4), substrate for:
• Leukotriene B4 (LTB4): Most potent neutrophil chemoattractant and activator; binds BLT1 receptor, triggering robust migration and respiratory burst. Elevated in asthma, ARDS, and inflammatory bowel disease.
• Cysteinyl leukotrienes (LTC4, LTD4, LTE4): Bind CysLT receptors on airway smooth muscle and bronchial blood vessels, causing bronchospasm, mucus secretion, and edema. Principal mediators of allergic asthma; leukotriene receptor antagonists (montelukast) provide clinical benefit. 12-LOX and 15-LOX in platelets, eosinophils, and epithelial cells produce other leukotrienes and lipoxins (pro-resolving mediators).

• Lipid Mediators of Resolution: Lipoxins, resolvins, and protectins (derived from arachidonic acid, EPA, and DHA via 5-LOX and other enzymes) are anti-inflammatory and pro-resolving, promoting neutrophil apoptosis, macrophage clearance of apoptotic cells, and tissue repair. Dysregulation → chronic unresolved inflammation in chronic obstructive pulmonary disease and cystic fibrosis.

COMPLEMENT SYSTEM DISORDERS

• Genetic Deficiencies: C1q, C2, C4 deficiencies (→ systemic lupus erythematosus-like disease); C3 deficiency (→ recurrent pyogenic infections + membranoproliferative GN); Factor H, Factor I deficiencies (→ atypical hemolytic uremic syndrome, C3 glomerulonephritis); C1-INH deficiency (→ hereditary angioedema). Properdin deficiency (→ meningococcal infection susceptibility).

• Acquired Dysregulation: Immune complex deposition (SLE, post-infectious GN, serum sickness) overwhelms capacity for soluble complement factor control. C3 nephritic factors (autoantibodies stabilizing C3 convertase) → C3 glomerulonephritis. Streptococcal infection directly activates alternative pathway. Endotoxemia (LPS) triggers all three pathways.

CYTOKINE-MEDIATED DISEASE

• Sepsis/SIRS: Polymicrobial or monomicrobial infection → massive TNF-α, IL-1, and IL-6 release → SIRS cascade (fever, tachycardia, hyperventilation, leukocytosis) progressing to septic shock (profound hypotension from iNOS-mediated vasodilation), disseminated intravascular coagulation (DIC), and multi-organ failure.

• Autoimmune/Inflammatory Disease: Dysregulated Th1/Th17 responses with excess TNF-α, IL-6, IL-17, IFN-γ → rheumatoid arthritis, inflammatory bowel disease, psoriasis. TNF-α blockade (infliximab, etanercept) and IL-6 antagonism effective therapies.

• Allergic/Immediate Hypersensitivity: IgE cross-linking on mast cells → TNF-α, IL-4, IL-5, and IL-13 release ��� anaphylaxis, allergic rhinitis, atopic dermatitis. IL-4/IL-13 promote IgE class switching (autocrine amplification); IL-5 recruits eosinophils (key in asthma pathogenesis).

ARACHIDONIC ACID-MEDIATED CONDITIONS

• Asthma: Th2-driven airway inflammation with elevated cysteinyl leukotrienes and PGD2; aspirin sensitivity (aspirin → shunting to LOX pathway, excess LT production).

• Inflammatory Bowel Disease (IBD): Dysbiotic microbiota → aberrant IL-17 and TNF-α production by lamina propria T cells + elevated LTB4; 5-ASA compounds inhibit NF-κB and reduce leukotriene production.

• Cardiovascular Disease: Atherosclerosis-associated inflammation with TXA2-PGI2 imbalance (prothrombotic state); TNF-α and IL-6 promote endothelial activation and plaque rupture.

ACUTE INFLAMMATION (LOCAL)

• Cardinal Signs: Calor (heat), rubor (redness), tumor (swelling), dolor (pain) mediated by PGE2-induced vasodilation, histamine/bradykinin-driven vascular permeability, and C5a/IL-8-mediated neutrophil infiltration. Functio laesa (loss of function) from pain and tissue edema.

• Neutrophil Infiltration: C5a and IL-8 chemotaxis drives margination and extravasation (mediated by ICAM-1/LFA-1 and selectin interactions). Neutrophils undergo respiratory burst (NADPH oxidase activation → O2•−, H2O2, •OH) and degranulation (proteases, lysozyme, lactoferrin), attacking pathogens but also causing local tissue damage (especially in acute pancreatitis and ARDS).

• Morphological Correlate: Histology shows acute inflammatory infiltrate (polymorphonuclear cells + some macrophages), fibrin deposition, vascular dilation and congestion, and edema fluid.

SYSTEMIC INFLAMMATION (SEPSIS/SIRS)

• Fever: TNF-α and IL-1β induce hypothalamic PGE2 synthesis via COX-2 upregulation; EP3 receptor signaling resets thermoregulatory set-point → elevated core temperature.

• Shock: Overwhelming TNF-α, IL-1, IL-6 release → iNOS induction in vascular smooth muscle (excessive NO production causing refract

Screening the complement axis

• CH50 and AH50 (functional hemolytic assays): the initial tests. CH50 interrogates classical + terminal components; AH50 interrogates alternative + terminal. An undetectable CH50 in a patient with recurrent Neisseria infection is the classic screen for a terminal component (C5–C9) deficiency. Both assays abnormal → shared terminal pathway; CH50 alone abnormal → C1/C2/C4; AH50 alone abnormal → factor B, factor D, or properdin.
• C3 and C4 levels: pattern recognition drives the differential. Low C3 with low C4 indicates classical/immune-complex consumption (SLE, cryoglobulinemia, serum sickness); low C3 with normal C4 indicates alternative-pathway consumption (post-streptococcal GN, C3 glomerulopathy).
• Atypical HUS caveat: C3 may be low, but it is normal in many patients — normal complement levels do not exclude aHUS. Diagnosis rests on the clinical thrombotic microangiopathy picture (Coombs-negative hemolysis with schistocytes, thrombocytopenia, renal failure, normal ADAMTS13) plus regulator genetics and anti-factor H antibodies.
• Confirmatory testing: individual component antigenic and functional assays, C3 nephritic factor, anti-factor H antibodies, and genetic sequencing of regulators (CFH, CFI, MCP).
• Hereditary angioedema: C4 is low both during and between attacks and is the best screening test; confirm with C1-inhibitor antigen (low in type I) and C1-INH function (low in type II despite normal antigen). A low C1q suggests acquired C1-INH deficiency from lymphoproliferative disease. This stepwise algorithm follows the WAO/EAACI angioedema guideline.

Cytokine and eicosanoid surrogates

• Acute-phase reactants: CRP, ESR, fibrinogen, and ferritin are IL-6–driven downstream readouts; IL-6 itself is rarely measured outside research. Procalcitonin supports bacterial etiology but cannot rule sepsis in or out alone.
• Sepsis: diagnosed by the Sepsis-3 definition — suspected infection plus an acute increase in SOFA score of ≥2 points over baseline. qSOFA ≥2 (altered mentation, systolic BP ≤100 mmHg, respiratory rate ≥22/min) is a bedside screening prompt, not a diagnostic standard. Lactate and repeat lactate are mandated by the Surviving Sepsis Campaign bundle.
• Anaphylaxis: a clinical diagnosis using the NIAID/FAAN criteria. Serum tryptase drawn within a few hours of onset supports mast-cell degranulation; a persistently elevated baseline tryptase suggests mastocytosis or hereditary alpha-tryptasemia.
• Arachidonic acid disorders: urinary LTE4 is elevated in aspirin-exacerbated respiratory disease; graded oral aspirin challenge under supervision remains the gold standard for confirming AERD per AAAAI/ACAAI practice parameters.

Immediate stabilization

• Epinephrine (alpha-1 vasoconstriction plus beta-2 bronchodilation and mast-cell stabilization): epinephrine 0.3 mg IM into the anterolateral thigh is first-line for anaphylaxis, repeated every 5–15 minutes as needed, per the AAAAI/ACAAI Joint Task Force. Antihistamines and glucocorticoids are adjuncts only and must never delay epinephrine.
• Bradykinin-mediated angioedema is different: it does not respond to epinephrine, antihistamines, or steroids, and airway assessment comes first.
• Hereditary angioedema: treat acute attacks with plasma-derived or recombinant C1-inhibitor concentrate, the B2-receptor antagonist icatibant, or the kallikrein inhibitor ecallantide (US HAEA Medical Advisory Board / WAO-EAACI). Long-term prophylaxis uses lanadelumab or berotralstat.
• ACE-inhibitor angioedema: mainstays are airway management and permanent discontinuation of the ACE inhibitor; bradykinin-targeted agents are of unproven benefit, as randomized trials of icatibant in this setting were negative.

Cytokine-driven systemic inflammation

• Sepsis: the Surviving Sepsis Campaign directs broad-spectrum antimicrobials promptly (within one hour for septic shock), crystalloid resuscitation for hypoperfusion, and norepinephrine as first-line vasopressor. Anti-TNF and anti-endotoxin agents failed in sepsis trials and are not used — a favorite distractor.
• Chronic immune-mediated disease: ACR guidelines place methotrexate first for rheumatoid arthritis, escalating to a TNF inhibitor (adalimumab), IL-6 receptor antagonist (tocilizumab), or JAK inhibitor. Screen for latent tuberculosis and hepatitis B before any biologic; avoid live vaccines during therapy.
• IL-1 blockade (anakinra, canakinumab) is effective in autoinflammatory syndromes and refractory gout flares (ACR gout guideline).

Complement-directed therapy

• Anti-C5 monoclonal antibodies (eculizumab, ravulizumab) block MAC formation in PNH and atypical HUS. Meningococcal vaccination (MenACWY and MenB) is required before initiation; antibiotic prophylaxis is recommended, especially if treatment cannot be delayed 2 weeks after vaccination (CDC/ACIP) — MAC blockade recreates a terminal complement deficiency.

Eicosanoid modulation and contraindications

• NSAIDs/COX inhibitors for pain, fever, and inflammation; leukotriene modifiers (montelukast, zileuton) are add-on, not first-line, in asthma — inhaled corticosteroids remain the controller backbone per GINA/NAEPP. Montelukast carries an FDA boxed warning for neuropsychiatric events.
• Contraindicated: NSAIDs/aspirin in aspirin-exacerbated respiratory disease; aspirin in children with febrile viral illness (Reye syndrome); NSAIDs at or after 20 weeks' gestation (FDA warning — oligohydramnios, ductal constriction); NSAIDs in advanced CKD (KDIGO).

Complications of mediator dysregulation

• Neisserial sepsis and meningitis (EMERGENCY): terminal component (C5–C9) or properdin deficiency prevents MAC-mediated lysis of Neisseria, whose thin cell wall makes lysis the dominant defense. Signals: recurrent meningococcemia, absent CH50.
• Laryngeal edema in hereditary angioedema (EMERGENCY): unopposed bradykinin raises vascular permeability; voice change, stridor, or tongue swelling mandates immediate airway control plus C1-INH or icatibant.
• DIC, ARDS, and multiorgan failure in sepsis: TNF-α/IL-1 induce endothelial tissue factor and iNOS; falling platelets with rising D-dimer and prolonged PT signal DIC. Refractory hypotension despite fluids defines septic shock (EMERGENCY).
• Complement-mediated renal injury: uncontrolled alternative-pathway activation produces C3 glomerulopathy and atypical HUS — hemolytic anemia with schistocytes, thrombocytopenia, and rising creatinine.
• Fibrosis from chronic TGF-β signaling: unresolved inflammation drives pulmonary fibrosis and cirrhosis — restrictive PFTs with reduced DLCO.

Complications of therapy

• NSAID gastropathy: COX-1 blockade removes PGE2-mediated mucus/bicarbonate secretion and mucosal blood flow → ulceration and GI bleeding.
• NSAID acute kidney injury: loss of PGE2/PGI2 afferent arteriolar vasodilation collapses GFR in volume-depleted, cirrhotic, or heart failure patients; risk is highest with concurrent ACE inhibitor and diuretic. Also causes hyperkalemia and interstitial nephritis.
• Thrombotic risk with selective COX-2 inhibition: endothelial PGI2 falls while platelet TXA2 (COX-1) is preserved — a prothrombotic imbalance.
• Aspirin-exacerbated respiratory disease: COX inhibition shunts arachidonate to 5-LOX → cysteinyl leukotriene surge with bronchospasm and nasal polyposis.
• Reye syndrome: aspirin in a febrile child → mitochondrial injury with encephalopathy and hepatic steatosis (EMERGENCY).
• TNF inhibitors: granuloma destabilization → reactivation tuberculosis, often disseminated or extrapulmonary; also histoplasmosis, demyelination, and drug-induced lupus.
• Tocilizumab: blunts CRP and fever, masking infection, and raises risk of GI perforation (diverticulitis) and transaminitis.
• Eculizumab/ravulizumab: boxed warning for fulminant meningococcal disease (EMERGENCY) — any fever demands immediate evaluation.

• C5a is the most potent chemotactic factor; C3a and C5a are the anaphylatoxins that trigger mast-cell degranulation. C5a plus LTB4 plus IL-8 is the classic neutrophil-chemotaxis triad — LTB4 is the lipid answer, IL-8 the chemokine answer.
• Complement pattern is the shortcut: low C3 with normal C4 = alternative pathway (post-streptococcal GN, C3 glomerulopathy); low C3 and C4 = classical/immune complex (SLE, cryoglobulinemia). Early classical deficiency (C1q, C2, C4) presents as SLE-like disease because immune complexes are not cleared. Normal complement does not exclude atypical HUS.
• Recurrent Neisseria infection → check CH50 first, then individual terminal components. The single best next step before starting eculizumab is meningococcal vaccination, since anti-C5 therapy reproduces a terminal complement deficiency.
• Angioedema without urticaria or pruritus is bradykinin-mediated, not histamine-mediated. Low C4 is the screen; epinephrine, antihistamines, and steroids will not work — the distractor examiners plant. C1-INH concentrate or icatibant is correct for hereditary angioedema; for ACE-inhibitor angioedema, stop the drug and protect the airway.
• Fever mechanism: IL-1β and TNF-α → hypothalamic COX-2 → PGE2 → EP3 receptor → raised thermoregulatory set-point. Antipyretics work by blocking COX, not by blocking the cytokine.
• Aspirin irreversibly acetylates COX-1, so platelets — lacking nuclei — cannot regenerate thromboxane A2 for their lifespan; this is why the antiplatelet effect outlasts the drug. In aspirin-exacerbated respiratory disease (asthma, nasal polyps, aspirin sensitivity), COX blockade shunts arachidonate down 5-LOX.
• PNH is a complement-regulation disease, not a complement-deficiency disease: a PIGA mutation strips the GPI anchor for CD55 and CD59, allowing MAC-mediated intravascular hemolysis. Diagnose with flow cytometry (FLAER), not the obsolete Ham test or sucrose lysis.
• Common distractor: TNF inhibitors are effective in rheumatoid arthritis and Crohn disease but failed in sepsis trials; anti-endotoxin strategies also failed in sepsis and have no role in autoimmune disease. The sepsis answer remains antibiotics, fluids, and norepinephrine.