Common complement component for both the pathways is the following:

Classic and alternate pathways lead to the production of C3b, the central molecule of the complement cascade. The presence of C3b on the surface of a microbe marks it as foreign and targets it for destruction. C3b has two impoant functions: (1) It combines with other complement components to generate C5 convease, the enzyme that leads to the production of the membrane attack complex; and (2) It opsonizes bacteria because phagocytes have receptors for C3b on their surface. Ref: Levinson W. (2012). Chapter 63. Complement. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.