The function of MASP1 (MBL associated serine protease 1, ENSG00000127241) is as follows. Precursor of a serum protease that activates the complement 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. {ECO:0000269|PubMed:10946292, ECO:0000269|PubMed:11485744, ECO:0000269|PubMed:11527969, ECO:0000269|PubMed:12538697, ECO:0000269|PubMed:22691502, ECO:0000269|PubMed:22966085, ECO:0000269|PubMed:23386610, ECO:0000269|PubMed:9087411}. [Isoform MASP-1]: Serine 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:10946292, PubMed:11527969, PubMed:12538697, PubMed:22691502, PubMed:22966085, PubMed:9087411). The lectin complement system is activated following association of lectins, such as MBL2, FCN1, FCN2 or FCN3, to carbohydrates on the pathogen surface (PubMed:22854970, PubMed:22966085, PubMed:23386610, PubMed:9087411). Following autoproteolytic processing in response to lectin-binding to pathogen carbohydrates, catalyzes cleavage and activation of MASP2, the next component of the lectin complement pathway (PubMed:22691502, PubMed:22966085, PubMed:23386610). {ECO:0000269|PubMed:10946292, ECO:0000269|PubMed:11527969, ECO:0000269|PubMed:12538697, ECO:0000269|PubMed:22691502, ECO:0000269|PubMed:22854970, ECO:0000269|PubMed:22966085, ECO:0000269|PubMed:23386610, ECO:0000269|PubMed:9087411}. [Isoform MASP-3]: Serine protease that activates the alternative 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:20038603, PubMed:23792966, PubMed:27535802). The alternative complement pathway acts as an amplification loop that enhances other complement pathways (PubMed:27535802). Isoform MASP-3 specifically catalyzes cleavage and activation of factor D (CFD), the protease that initiates the alternative complement pathway (PubMed:20038603, PubMed:27535802). {ECO:0000269|PubMed:20038603, ECO:0000269|PubMed:23792966, ECO:0000269|PubMed:27535802}.