C3 and COVID-19: However, taking into account that (a) the alternative complement pathway is more activated in high mortalin level patients, (b) the properdin level is reduced in high mortalin level patients, and (c) lower Bb is associated with a more severe disease, we hypothesize that COVID-19 patients have a more severe disease following activation of the alternative complement pathway, leading to a more stable fixation of the alternative pathway C3/C5 convertases C3bBbP and C3bBbC3bP on the affected tissue.