Extracellular lectin functioning as a pattern-recognition receptor in innate immunity. Binds the sugar moieties of pathogen-associated molecular patterns (PAMPs) displayed on microbes and activates the lectin pathway of the complement system. May also activate monocytes through a G protein-coupled receptor, FFAR2, inducing the secretion of interleukin-8/IL-8 (PubMed:21037097). 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. Homotrimer (PubMed:17897951). Interacts with elastin/ELN. Interacts (via Fibrinogen C-terminal domain) with FFAR2. Interacts with CRP; may regulate monocyte activation by FCN1. Found on the monocyte and granulocyte surface (PubMed:20400674). Peripheral blood leukocytes, monocytes and granulocytes. Also detected in spleen, lung, and thymus, may be due to the presence of tissue macrophages or trapped blood in these tissues. Not detected on lymphocytes. The fibrinogen C-terminal domain mediates calcium-dependent binding to carbohydrates and tethering to the cell surface in monocytes and granulocytes. The domain undergoes a conformational switch at pH under 6.2, and looses its carbohydrate-binding ability. Belongs to the ficolin lectin family.