The classical and lectin pathways of complement activation converge at the step of C3 activation. However, the initiating steps of each pathway use protein components and enzymatic mechanisms that share no similarity with each other.

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The classical and lectin pathways of complement activation both involve the formation of C3 convertases through a series of proteolytic cleavages, but they are initiated differently. The classical pathway is triggered by antibody-antigen complexes, while the lectin pathway is initiated by mannose-binding lectin or ficolins binding to specific sugars on the surface of pathogens. However, both pathways utilize similar protein components (C2 and C4) in the early steps to form the C3 convertase, indicating a similarity in their enzymatic mechanisms at this stage.

The Classical and Lectin Pathways of Complement Activation Converge at the Step