Opsonization takes place through-

1. All the three complement activation pathways(classical , alternate , lectin) lead to activation of C3, resulting in the production of C3b. Hence, C3b is considered as the central molecule in the activation of the complement cascade. 2. The C3b has two impoant functions to perform First, it combines with other components of the complement system to produce C5 convease, the enzyme that leads to the production of membrane attack complex(MAC) Second, it opsonizes bacteria due to the presence of receptors for C3b on the surface of the phagocytes. Biological Effects of Complement C5a: C5a is a chemotactic molecule specifically recognized by polymorphonuclear leukocytes or phagocytic cells. This substance causes leukocytes to migrate to a tissue in which an antigen-antibody reaction is taking place. At that site, a phagocytic cell recognizes opsonized paicles and ingests them.