Ficolins are a group of proteins containing both a collagen-like domain and a fibrinogen-like domain and are found in varying tissues. Ficolins present in sera have a lectin activity toward N-acetylglucosamine through their fibrinogen-like domains. The domain organizations between ficolins and mannose-binding lectin (MBL) are very similar in that both consist of a collagen-like domain and a carbohydrate-binding domain, although their carbohydrate-binding moieties are different. MBL acts as an opsonin and activates complement in association with MBL-associated serine proteases (MASPs) and sMAP, truncated protein of MASP-2, via the lectin pathway. Like MBL, two types of human serum ficolins, L-ficolin/P35 and H-ficolin, are associated with MASPs and sMAP, and activate the lectin pathway. In addition, L-ficolin/P35 acts as an opsonin. These findings indicate that serum ficolins play an important a role in innate immunity in a similar manner to MBL.