15. Lectin and Classical Complement Pathways Explained

Two of complement's three doors: the lectin and classical pathways. MBL and ficolins with their MASPs, or C1q with C1r and C1s, recognise a microbe, cut C4 and C2, and build the same C3 convertase, C4b2a, which coats the pathogen in C3b. Episode 15 of Primer on Immunology. In this episode • PAMPs: lipoteichoic acid and lipopolysaccharide • Sugar patterns: mannose on yeasts, sialic acid on our own cells • Mannose-binding lectin: monomer, trimer, bouquet; collectins • Affinity vs avidity: why MBL holds microbes and ignores host cells • Ficolins and the acute-phase response • MASP-1, MASP-2, MASP-3 (and factor D) • Building C4b2a step by step: C4, C2, then a shower of C3b • MBL and MASP-2 deficiency, and the early-childhood gap • Surfactant proteins A and D • The C1 complex: C1q, C1r, C1s — "a bunch of tulips" • What C1q binds: bacterial surfaces, C-reactive protein, antibody Fc • Natural IgM, before any infection • Why complement stays on the pathogen surface • Antibody plus complement: the strongest trigger for phagocytosis Chapters 0:00 Welcome 0:11 Two doors, one machine 0:23 Patterns: PAMPs 0:56 Mannose vs sialic acid 1:22 Mannose-binding lectin 2:08 Affinity vs avidity 2:34 What MBL binds 2:56 Ficolins 3:30 The MASPs 4:21 Building C4b2a, step by step 5:32 Why it matters: early childhood 6:02 Surfactant proteins A and D 6:21 The classical pathway: C1 6:51 The same machine 7:30 What C1q binds 8:07 Natural antibodies 8:52 Complement stays where it starts 9:26 Antibody plus complement 9:45 Summary, and next: the alternative pathway Next: the third door — the alternative pathway, always switched on. Everything we make is at rheumatologydigest.org WhatsApp channel (new material first): https://whatsapp.com/channel/0029Vb6P... For educational purposes only. Rheumatology Digest is made for students and clinicians; it is not medical advice. #immunology #complement #innateimmunity