The classical complement pathway is initiated by C1q binding to the surface of a pathogen. In some cases, C1q can directly bind the pathogen, for instance by recognizing proteins of bacterial cell walls, but in most cases C1q binds to IgM antibodies that are bound to the pathogen surface. How does this IgM-binding feature of C1q contribute to rapid, innate immune responses rather than to slow, adaptive responses?
A) C1q induces B lymphocytes to begin secreting antibody within hours of pathogen exposure.
B) Natural antibody that binds to many microbial pathogens is produced prior to pathogen exposure.
C) C1q binds to C-reactive protein which then binds to IgM on the pathogen surface.
D) C1q directly induces inflammation, recruiting phagocytes and antibodies from the blood into the infected tissue.
E) C1q binds to dendritic cells in the infected tissue, inducing them to secrete inflammatory cytokines.

Question

Quizplus · Accepted Answer

The presence of natural IgM antibodies that can bind to a broad range of microbial pathogens allows for immediate activation of the classical complement pathway upon pathogen exposure. This is because C1q can directly bind to the pathogen-bound IgM, without the need for the slower adaptive response of generating pathogen-specific antibodies.

The Classical Complement Pathway Is Initiated by C1q Binding to the Surface