Final answer:
The key immunologic substance against bacteria is Complement, which helps in directly lysing bacterial cells or marking them for destruction by other immune cells.
Step-by-step explanation:
A key immunologic substance against bacteria is Complement. Complement is a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promotes inflammation, and attacks the pathogen's cell membrane. This mode of action is different from interferons, which are particularly useful against viral infections, and from lysozyme, which is an enzyme that breaks down the cell walls of bacteria. Interleukins are associated primarily with immune responses such as the production of various lymphocytes. However, when looking at direct effectiveness against bacteria, complement proteins play a significant role, either by directly lysing bacterial cells or by marking them for destruction by other immune cells.