C3 and rheumatoid arthritis: Taken together, we may speculate that C3 appeared to be increased in OA compared to healthy controls, but it decreases when OA progresses because the activation of complement cascade could be the main driver of inflammation at the first stages of the OA but at end stages (K/L = 4), this inflammatory pathway might switch to other biochemical pathways more associated to advanced OA and chronic arthritis such as RA.