Final answer:
Soluble innate immune proteins, making up the complement system, tag pathogens for destruction rather than protection. They act in conjunction with the adaptive immune system to enhance pathogen elimination through a highly regulated binding cascade.
Step-by-step explanation:
Soluble innate immune proteins in the blood, particularly the complement system, play a crucial role in defending the body against pathogens by tagging them for destruction. The complement system, consisting of about 20 proteins, functions in a cascade to identify and eliminate extracellular pathogens efficiently. Proteins synthesized mainly by liver and macrophages are persistent in the blood serum, ready to act against invading microorganisms. Complement proteins bind in a highly regulated sequence to the surfaces of pathogens, especially those already bound by antibodies produced by the adaptive immune system. This process facilitates the pathogen's elimination via opsonization, which improves phagocytosis by immune cells like macrophages.