What are the three pathways through which the complement system can be activated?

• A. Classical, Alternative, Lectin
• B. Innate, Adaptive, Cytokine
• C. Inflammatory, Cellular, Humoral
• D. Direct, Indirect, Pathogen-based

Answer: (A)

The complement system can be activated through three specific pathways, which are the classical pathway, the alternative pathway, and the lectin pathway.

The classical pathway is initiated when antibodies bind to an antigen, which then triggers a series of complement proteins to assemble and create a cascade of biological reactions aimed at eliminating the pathogen. The alternative pathway can activate in the absence of antibodies, relying instead on the direct interaction of complement proteins with pathogen surfaces. This pathway provides a rapid response to infections. The lectin pathway is similar to the classical pathway but is triggered by the binding of mannose-binding lectin (MBL) to specific sugars on the surface of pathogens, leading to the same complement cascade.

Understanding these pathways is crucial as they demonstrate the versatility and fundamental role of the complement system in the immune response, both in innate and adaptive immunity. Each pathway contributes to opsonization, cell lysis, and inflammation, highlighting their importance in fighting infections and maintaining immune health.