This gene encodes complement protein C3 which plays a central role in the classical, alternative and lectin activation pathways of the complement system. The encoded preproprotein undergoes a multi-step processing to generate various functional peptides. Mice deficient in the encoded protein fail to clear bacteria from the blood stream upon infection, display diminished airway hyperresponsiveness and lung eosinophilia upon allergen-induced pulmonary allergy, and develop severe lung injury after deposition of IgG immune complexes. Deficiency of the homolog of the encoded protein in humans was found to be associated with increased susceptibility to infections, age-related macular degeneration, and atypical hemolytic uremic syndrome. [provided by RefSeq, Mar 2015]