The C3 convertase of the classical and Lectin pathways is the same **C4B2a** The C3 convertase of the alternate pathway is made of C3b and factor B (remember C3 is auto-cleaved by tissue damage or microbial products to start this pathway). Factor D cleaves Factor B which combines with C3b to form C3 convertase. Factor B is a soluble factor circulating in the blood and is produced by the liver and other tissues of the body (https://pmc.ncbi.nlm.nih.gov/articles/PMC11868072/). {{ :general_pathology:acute:complement3.png |}} {{ :general_pathology:acute:complement_factorb.png |}} From: https://pmc.ncbi.nlm.nih.gov/articles/PMC11868072/ Also see: https://en.wikipedia.org/wiki/Complement_factor_B