Final answer:
The main opsonin (complement) that is placed on pathogen surfaces to facilitate their recognition and destruction by the immune system is C3b.
Step-by-step explanation:
The main opsonin (complement) placed on pathogen surfaces is C3b. The complement system, which includes a sequence of protein components, plays a crucial role in the immune response to infections. When the C3 protein of the complement system splits, the larger fragment, C3b, binds to the surface of the pathogen, acting as an opsonin, which facilitates the process of phagocytosis by phagocytic cells such as macrophages and neutrophils. This attachment enhances the ability of immune cells to recognize, attach to, and eventually engulf and destroy the pathogen. The process of complement activation can be triggered via the classical or alternative pathway, but ultimately, it's C3b that tags the pathogens for destruction.