Similar to classical pathway, but involves lectin. Lectin, an acute-phase protein, triggers the cascade by binding to carbohydrates on the microbial surface.

University of Notre Dame

Three pathways that trigger complement activation include:
- Alternative pathway
- Classical pathway
- Lectin pathway

(Photo credits in References)

Pathways Triggering Complement Activation

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Pathways Triggering Complement Activation Figure (Figure 17.9). Reprinted from OpenStax Microbiology (p. 696), by N. Parker, M. Shneegurt, A.-h. Thi Tu, B. M. Forester, and P. Lister, 2016, Rice University. Copyright 2016 by Rice University.

Pathways Triggering Complement Activation Figure

Initiated by the spontaneous activation of complement protein C3, the hydrolysis of which forms C3a and C3B. With invaders absent: C3B degraded in a hydrolysis reaction. Without invaders present: C3b attaches to invaders and recruits other complement proteins.

Complement Activation: Alternative Pathway

Can be more efficient when activated by specific antibody produced by the adaptive immune response. The specific antibody attaches to the pathogen to form an antibody-antigen complex, activating mutlipart protein C1. C1 then recruits other complement proteins in a cascading manner.

Complement Activation: Classical Pathway

Complement Activation: Lectin Pathway

Despite the differences in complement activation pathways, they all share and provide four protective outcomes: opsonization, inflammation, chemotaxis, and cytolysis.

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