Lectin Pathway (LP)
It is important to realize that the LP is identical to the Classical pathway (CP) except for the components involved in the initiation of the complement cascade. Instead of a C1q/C1 complex the LP initiation is done by an opsonin, mannos-binding lectin (MBL) or ficolins. MBL binds to mannose residues on the pathogen surface which in turn activates the associated serine proteases MASP1 and MASP2. MBL may be replaced by ficolins which act in an identical way together with MASPs binding to carbohydrate moieties on the pathogen surface. There are several forms of ficolins, referred to as M-ficolin, L-ficolin, and H-ficolin, respectively. In the circulation the most abundant ficolin is H-ficolin also called ficolin 3 or Hakata antigen. The MBL/ficolin in complex with MASPs can cleave C4 into C4a and C4b and C2 into C2a and C2b, respectively. Functionally, C4b and C2b forms the C3 convertase (C4b2a) identical to the one formed in the CP. The C3 convertase later joins with C3b to make the C5 convertase (C4b2a3b). For more details on the cascade see Classical pathway (CP).
The Illustration shows the Lectin pathway. Additional information can be found if you place the cursor on the different parts of the illustration.