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.

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

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

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

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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