Three Complement Activation Pathways
The complement cascade can be activated by three convergent pathways: (1) Classical pathway: IgG/IgM bound to antigen triggers C1q/C1r/C1s recognition, then C4/C2 cleavage forming the classical C3 convertase (C4b2a); (2) Lectin pathway: MBL (mannose-binding lectin) or ficolins recognise carbohydrate patterns, activating MASPs (MBL-associated serine proteases) to cleave C4/C2; (3) Alternative pathway: spontaneous C3 hydrolysis (tick-over) forms C3(H2O)-Bb convertase; properdin (CFP) stabilises C3bBb on activating surfaces. All three converge on C3 cleavage into C3a (anaphylatoxin) and C3b (opsonin deposited on targets).
C3b Deposition and Opsonisation
C3b covalently deposits on pathogen surfaces via thioester bond formation with hydroxyl/amine groups. C3b is recognised by complement receptor 1 (CR1/CD35) on phagocytes, facilitating phagocytosis. iC3b (from CR1/Factor I cleavage of C3b) is recognised by CR3 (Mac-1/CD11b-CD18, integrin) for phagocytosis without oxidative burst. C3b further amplifies the cascade: C3b-Bb (alternative C3 convertase) plus additional C3b forms the C5 convertase (C3b2Bb or C4b2a-C3b), cleaving C5 into C5a (most potent anaphylatoxin, CR3/CXCR1/2 signalling) and C5b (MAC initiation).
Membrane Attack Complex (MAC)
C5b sequentially recruits C6, C7, C8, and multiple C9 molecules to form the MAC (C5b-C9), inserting into lipid bilayers and forming transmembrane pores (~10 nm diameter). MAC directly lyses gram-negative bacteria, spirochaetes, and complement- susceptible cells. In regulation, complement regulatory proteins prevent MAC on host cells: CD59 (protectin) blocks C9 polymerisation; CD55 (DAF, decay-accelerating factor) accelerates C3 convertase decay; CD46 (MCP) is a cofactor for Factor I- mediated C3b cleavage. C-reactive protein (CRP) activates the classical pathway (C1q binding), and spirulina's documented CRP reduction links to reduced complement-mediated inflammatory activation.
Spirulina Polysaccharides and Complement Activation
Sulfated polysaccharides from spirulina (spirulan, calcium spirulan) interact with the complement system in a dose-dependent, pathway-specific manner. At low doses, spirulan can act as a complement activator via the lectin pathway (mannose residues engaging MBL), enhancing opsonisation and phagocyte recruitment. At high doses, sulfated polysaccharides inhibit C3 convertase by direct C3 binding competition (similar to heparin), reducing excessive complement activation. This biphasic dose-response mirrors spirulina's broader immunomodulatory (rather than purely immunostimulatory or immunosuppressive) profile.
C3a/C5a Anaphylatoxins and Inflammation
C3a and C5a are potent pro-inflammatory peptides binding C3aR and C5aR1/C5aR2 (the latter being a decoy anti-inflammatory receptor). C5a-C5aR1 activates Gq/Gi in mast cells (histamine/tryptase release), macrophages (TNF-alpha, IL-6), and neutrophils (respiratory burst, NET formation, degranulation). Dysregulated complement activation (as in COVID-19 coagulopathy, atypical HUS, lupus) is a major driver of severe inflammation. Spirulina's NF-kB suppression attenuates C5a-induced inflammatory cytokine production in downstream cells, while sulfated polysaccharides may buffer excessive C3 convertase activity.
NLRP3 and Complement Cross-talk
C5a-C5aR1 primes NLRP3 inflammasome activation (Signal 1 via NF-kB for pro-IL-1beta and NLRP3 transcription), and C5a potentiates GSDMD-mediated IL-1beta release. C1q binds LAIR-1 (Leukocyte-associated immunoglobulin-like receptor-1) on macrophages, inhibiting NLRP3. Spirulina thus attenuates the complement-NLRP3 amplification axis through: (1) PCB-NF-kB suppression reducing C5a-driven NLRP3 priming; (2) TXNIP suppression reducing NLRP3 Signal 2 licensing; (3) sulfated polysaccharides moderating C5 cleavage and C5a generation.
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