Extracellular lectin, which acts as a pattern recognition receptor that initiates the lectin pathway of the complement system, a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system (PubMed:16116205, PubMed:17897951, PubMed:20032467). Specifically recognizes and binds carbohydrates on the pathogen surface, activating the MASP1 serine protease and initiating the proteolytic cascade of the lectin complement pathway (PubMed:16116205, PubMed:20032467). Binds preferentially to 9-O-acetylated 2-6-linked sialic acid derivatives and to various glycans containing sialic acid engaged in a 2-3 linkage (PubMed:20032467, PubMed:20400674). May also activate monocytes through a G protein-coupled receptor, FFAR2, inducing the secretion of interleukin-8/IL-8 (PubMed:21037097). {ECO:0000269|PubMed:16116205, ECO:0000269|PubMed:17897951, ECO:0000269|PubMed:20032467, ECO:0000269|PubMed:20400674, ECO:0000269|PubMed:21037097}. This is the function of FCN1 (ficolin 1, ENSG00000085265).