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. 2023 Nov 8;14:1297352. doi: 10.3389/fimmu.2023.1297352

Figure 1.

Figure 1

Classical, lectin, and alternative pathways of the complement system. The classical, lectin, and alternative pathways of complement converge at C3, which is cleaved to activate C5 and initiate formation of the membrane attack complex (C5b-9). The lectin pathway is activated in response to patterns present on pathogens and injured host cells. MASP-2 is the key effector enzyme of the lectin pathway and is activated following surface sequestration via associated PRMs (MBL, ficolins-1, 2, and 3, collectin-10, and collectin-11) binding to carbohydrate and aberrant glycosylation patterns on pathogens and injured host cells. Activated MASP-2 cleaves C4 and C2, resulting in the formation of the C3 convertase C4b2b, which activates C3. MASP-2 also activates the coagulation cascade through cleavage of prothrombin to thrombin and activation of Factor XII. Adapted from Gavriilaki E et al. Exp Hematol Oncol 2021; 10:57. MAC, membrane attack complex (C5b-9); MASP, mannan-binding lectin-associated serine protease; MBL, mannan-binding lectin; PRM, pattern-recognition molecule.