The complement system is a well-characterised cascade of extracellular
serum proteins that is activated by pathogens and unwanted waste
material. Products of activated complement signal to host cells via
cell-surface receptors, illicting responses such as removal of the
stimulus by phagocytosis. The complement system therefore functions as a
warning system, resulting in removal of unwanted material. This review
describes how extracellular activation of the complement system can also
trigger autophagic responses within cells, upregulating protective
homeostatic autophagy in response to perceived stress, but also
intiating targeted anti-microbial autophagy in order to kill
intracellular cyto-invasive pathogens. In particular, we will focus on
recent discoveries that complement may also have roles in detection and
autophagy-mediated disposal of unwanted materials within the
intracellular environment. We therefore summarize the current evidence
for complement involvement in autophagy, both by transducing signals
across the cell membrane, as well as roles within the cellular
environment.