Ficoline

Ficolin is a family of soluble pattern-recognition receptors that participate in the lectin pathway of the complement system. In humans, three members are known: ficolin-1 (M‑ficolin), ficolin-2 (L‑ficolin), and ficolin-3 (H‑ficolin). These proteins are encoded by the FCN1, FCN2, and FCN3 genes, respectively. Ficolin-1 is produced by myeloid cells, while ficolin-2 and ficolin-3 are mainly synthesized by the liver and circulate in the bloodstream.

Structurally, each ficolin has a collagen-like domain at the N-terminus and a C-terminal fibrinogen-like domain that mediates ligand recognition. They assemble into oligomeric complexes in plasma. Ligand binding occurs primarily to acetylated compounds and certain carbohydrate structures found on microbial surfaces and damaged cells.

Functionally, ficolins act as initiators of the lectin pathway by associating with MASP serine proteases (MASP-1, MASP-2, and MASP-3). This interaction leads to activation of the complement cascade, including C4 and C2 cleavage, formation of the C4b2a convertase, and downstream opsonization and inflammatory responses. In addition to pathogen recognition, ficolins can bind to apoptotic cells, contributing to clearance and immune regulation.

Clinical and research relevance includes genetic and environmental factors that influence circulating ficolin levels and function. Variants in FCN1, FCN2, and FCN3, as well as altered ficolin levels, have been linked to susceptibility to infections and various inflammatory or autoimmune conditions. Ficolins are covered in immunology research as contributors to innate immunity and potential biomarkers of disease.