For anyone unfamiliar., the complement system is a bit like the way some kids play tag. When antibodies attach to whatever they're programmed to attack, they may activate a cascade, which starts with C3 and C4, then attaches to the target cell and starts a process that makes a hole in the membrane of that cell and so destroy it. When things work properly, the destroyed cell is a bacteria; in autoimmune disease, it's 'self'. (Given that this is an evolutionarily early system, that might be the original own-goal).

BTW/FWIW, low serum C3 is one of the markers for lupus (which I belive might also strike XX-havers disproportionately). Go figure.

Also, I am writing this on a mobile. The editor is spectacularly unwieldy. Apologies for typos.