The innate immune system contributes to the development of rheumatoid arthritis (RA). A potent contributor to such processes is the complement system. The complement system is known to be activated in the inflammatory phases of osteoarthritis (OA). The lectin pathway of the complement system is activated through the recognition of pathogens or altered self-structures by mannan-binding lectin (MBL) or one of the three ficolins in collaboration with MBL-associated serine proteases (MASPs). We assessed the lectin pathway in plasma and synovial fluid (SF) of 27 RA patients and 30 OA patients by measuring MBL, MASP-2, MASP-3, M-ficolin, and H-ficolin. The concentration for all 5 proteins was significantly higher in plasma than in SF (P