Which of the components of the innate immune system coat the surface of invading organisms to facilitate their attack by macrophages?

• A. MHC proteins
• B. Intracellular vesicles
• C. Antibodies
• D. Complement proteins

Answer: (D)

Opsonization of pathogens by components of the innate immune system is the key idea here. Complement proteins circulate in the blood and, when activated, bind to the surface of invading organisms. This coating marks the microbes and makes them much easier for macrophages to recognize and engulf, effectively tagging them for attack. Among the options, complement proteins are the ones specifically responsible for this surface coating that enhances phagocytosis.

Antibodies are part of the adaptive immune response, created by B cells, and they also can coat pathogens but through a different, learned process, not the innate pathway described. MHC proteins are involved in presenting antigen fragments to T cells, not coating pathogens for macrophage attack. Intracellular vesicles relate to cellular transport and processing within cells, not coating external invaders.