How many proteins are used in both the classical and alternative pathways of the complement system?

The complement system is a critical part of the immune response, with both classical and alternative pathways contributing to pathogen elimination. In both pathways, certain proteins function similarly to facilitate the opsonization of pathogens, promote inflammation, and enhance phagocytosis.

The correct answer identifies that six proteins are common to both the classical and alternative pathways. These proteins include C1, C2, C3, C4, C5, and Factor B, which play various roles in cascade reactions leading to the activation of the complement system. Specifically, C3 is central to both pathways, while C1 is specific to the classical pathway, and Factor B is specific to the alternative pathway, but all six proteins are involved in the overall complement activation process.

This commonality helps underscore the integrated functions of the complement system across different immune pathways, allowing for a more robust and versatile response to infections. Understanding the overlap in these proteins is vital for comprehending how the immune system can effectively combat pathogens through multiple mechanisms.