Editing Complement system (section)

=== Lectin pathway ===
{{Main|Lectin pathway}}
The [[lectin]] pathway is homologous to the classical pathway, but with the opsonin, [[mannose-binding lectin]] (MBL), and [[ficolin]]s, instead of C1q. This pathway is activated by binding of MBL to mannose residues on the pathogen surface, which activates the MBL-associated serine proteases, [[MASP1 (protein)|MASP-1]], and [[MASP2 (protein)|MASP-2]] (very similar to [[Complement component 1r|C1r]] and [[C1s]], respectively), which can then split C4 into [[C4a]] and [[C4b]] and C2 into [[C2a]] and [[C2b]]. C4b and C2b then bind together to form the classical [[C3-convertase]], as in the classical pathway. Ficolins are homologous to MBL and function via MASP in a similar way. Several [[single-nucleotide polymorphism]]s have been described in M-ficolin in humans, with effect on ligand-binding ability and serum levels. Historically, the larger fragment of C2 was named C2a, but it is now referred to as C2b.<ref>{{Cite journal |display-authors=6 |vauthors=Ammitzbøll CG, Kjær TR, Steffensen R, Stengaard-Pedersen K, Nielsen HJ, Thiel S, Bøgsted M, Jensenius JC |year=2012 |title=Non-synonymous polymorphisms in the FCN1 gene determine ligand-binding ability and serum levels of M-ficolin |journal=PLOS ONE |volume=7 |issue=11 |pages=e50585 |bibcode=2012PLoSO...750585A |doi=10.1371/journal.pone.0050585 |pmc=3509001 |pmid=23209787 |doi-access=free}}</ref> In invertebrates without an adaptive immune system, ficolins are expanded and their binding specificities diversified to compensate for the lack of pathogen-specific recognition molecules.{{citation needed|date=July 2022}}