The airway epithelium, from the nasal cavity to the alveoli, persistently encounters toxins and
pathogens. If the invader clears the mucus and breaches the epithelium, it must be
contained by an innate immune system while awaiting arrival of the adaptive immune
system. Hence, we need a biological process to immediately recognize, engage, and
destroy a pathogen. The complement cascade is the prototype of such an innate process. It
quickly amplifies based on cleavage of its most abundant complement protein: C3 to C3a …