FCN1 encodes ficolin-1 (M-ficolin), an extracellular lectin that functions as a pattern recognition receptor initiating the lectin pathway of complement 12. FCN1 specifically recognizes and binds carbohydrates on pathogen surfaces, particularly 9-O-acetylated sialic acids and 2-3-linked sialic acid glycans, activating the MASP1 serine protease to trigger the proteolytic complement cascade 23. This mechanism promotes pathogen phagocytosis and adaptive immune signaling. Additionally, FCN1 may activate monocytes via FFAR2, inducing interleukin-8 secretion 4. Genetic variation in FCN1 significantly impacts M-ficolin serum levels and function; the -144C>A promoter polymorphism increases M-ficolin concentration by 26%, while non-synonymous mutations (Ser268Pro, Ala218Thr, Asn289Ser) reduce protein production or ligand-binding capability 5. Clinically, FCN1+ myeloid populations are emerging as important biomarkers in multiple diseases. FCN1+ tumor-associated macrophages may induce inflammation in solid tumors 6, while CD14+FCN1hi macrophages serve as triggers and amplifiers of excessive S100A8/A9 inflammation in severe COVID-19 7. In tuberculosis infection, MM-FCN1 macrophages associate with disease status 8, and FCN1+ macrophages contribute to systemic sclerosis-related lung fibrosis pathogenesis 9. FCN1 also represents a glycolysis-related diagnostic signature for diabetic nephropathy 10.