14. Complement System - Overview and Pathways

Complement is a cascade of more than 30 plasma proteins that recognise microbes, amplify the alarm and kill. This episode gives you the whole map: zymogens and activation by cutting, the three pathways, the cleavage of C3, the thioester "staple gun", the convertases, and the three effector arms: inflammation, phagocytosis and membrane attack. Episode 14 of Primer on Immunology. In this episode • Jules Bordet and the heat-labile factor that "complements" antibody • Complement evolved first, as part of innate immunity • Zymogens, proteases and the amplifying cascade • The colour-coded map: recognition, amplification, then inflame, engulf or puncture • Three pathways: classical (C1), alternative (spontaneous C3), lectin (MBL and ficolins) • Names: C1–C9, the a and b fragments, the C2a exception, C1q/C1r/C1s, factor B • The cleavage of C3: C3a and C3b • The thioester: how C3b bonds covalently, and why water switches it off • Opsonisation, the C5 convertase and the membrane-attack complex • The four convertases: C4b2a, C3bBb, C4b2a3b, C3b2Bb • Control, and the clearance of dying cells • How complement shapes adaptive immunity Chapters 0:00 Welcome 0:14 Into the blood 0:38 Bordet and the history 1:14 Inactive proteins, activation by cutting 1:31 Zymogens and the cascade 2:13 The colour-coded map 2:49 Three pathways 3:32 Names: C1 to C9 3:57 Fragments, C1 and factor B 4:40 The cleavage of C3 5:14 The thioester: a loaded staple gun 6:02 C3b and the three effector arms 6:48 The four convertases 7:15 Control and dying cells 7:56 Complement and adaptive immunity 8:22 The whole map 8:34 Next: the lectin and classical pathways, and links Next: the first two doors into complement, the lectin and classical pathways. Everything we make is at rheumatologydigest.org WhatsApp channel (new material first): https://whatsapp.com/channel/0029Vb6P... Icons from Noun Project (CC BY 3.0): Liver by Ashok. For educational purposes only. Rheumatology Digest is made for students and clinicians; it is not medical advice. #immunology #complement #innateimmunity