Which of the following plays an important role as an external defense mechanism?

• A. Phagocytosis
• B. C-reactive protein
• C. Lysozyme
• D. Complement

Answer: (C)

External defense mechanisms are the chemical and physical barriers at body surfaces that act before pathogens invade tissues. Lysozyme fits this role because it’s an antimicrobial enzyme found in secretions like tears, saliva, and mucus, where it directly acts on bacteria outside the body. It cleaves the bonds in bacterial cell walls, helping kill or weaken bacteria before they can establish infection, particularly against Gram-positive organisms with thick peptidoglycan layers.

Phagocytosis represents an internal defense carried out by immune cells after a pathogen has breached barriers. C-reactive protein and the complement system are circulating internal defenses that function within the bloodstream and tissues, not as external barriers. Thus lysozyme is the best example of an external defense mechanism.