Figure 2.

Lectin pathway and alternative pathway of the complement system. Collectins (including MBL, CL-L1, CL-K1 and the complex of the latter CL-LK complex) recognize carbohydrates on surface of pathogens and activate the lectin pathway via MASP-1 and MASP-2. The alternative pathway is activated by spontaneous hydrolysis of C3. Factor D is a rate limiting factor for generation of the C3 convertase in the alternative pathway, which is activated by MASP-3. The alternative pathway works as an amplification loop for the lectin pathway.