Precursor of a serum protease that activates 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:11527969, PubMed:22691502). The lectin complement system is activated following association of lectins, such as MBL2, FCN1, FCN2 or FCN3, to carbohydrates on the pathogen surface (PubMed:22691502, PubMed:22966085). MASP2 is cleaved and activated by MASP1 in response to lectin-binding to pathogen carbohydrates (PubMed:10946292, PubMed:22949645, PubMed:22966085, PubMed:9087411). Can activate prothrombin to thrombin (PubMed:39924859). {ECO:0000269|PubMed:10946292, ECO:0000269|PubMed:11527969, ECO:0000269|PubMed:22691502, ECO:0000269|PubMed:22949645, ECO:0000269|PubMed:22966085, ECO:0000269|PubMed:39924859, ECO:0000269|PubMed:9087411}. [Mannan-binding lectin serine protease 2 B chain]: Serine protease component of the lectin complement pathway, which catalyzes cleavage and activation of C2 and C4, the next components of the complement pathway. {ECO:0000269|PubMed:10946292, ECO:0000269|PubMed:11527969, ECO:0000269|PubMed:22691502, ECO:0000269|PubMed:22949645, ECO:0000269|PubMed:22966085, ECO:0000269|PubMed:9087411}. This is the function of ENSG00000009724 (MASP2, MBL associated serine protease 2).