Explain the Complement activation: Three pathways?

• Classical pathway activated by antibodies bound to microbe (humoral arm of adaptive immunity). • Alternative pathway – complement protein activated on microbes (innate immunity). • Lectin pathway – activated when mannose-binding lectin (MBL) binds to microbes (innate immunity). Another protein – C-reactive protein (CRP) binds to phosphatidyl choline. Three pathways differ in activation but share downstream steps. C3 protein is cleaved into C3a and C3b; C3b is covalently attached to microbe, recruit subsequent complement cascade on the microbe surface.