Final answer:
The group of proteins designated by C and a number are part of the complement system, which plays a crucial role in the body's immune response by targeting and marking pathogens for destruction.
Step-by-step explanation:
The complement system involves numerous precursor proteins that circulate in plasma and become activated in a cascading sequence in the presence of microbes. This leads to the opsonization of pathogens, chemoattraction of leukocytes, induction of inflammation, and cytolysis. Complement proteins are synthesized primarily in the liver, and they play a critical role in both the innate and adaptive immune response by marking pathogens for destruction and assisting antibodies.