All of the following are protective mechanisms against bacteria except

• A. release of lipid A from the bacterial cell.
• B. production of antimicrobial defense peptides.
• C. phagocytosis.
• D. activation of complement.

Answer: (A)

Protective defenses against bacteria come from active host processes that kill or remove pathogens: antimicrobial peptides disrupt bacterial membranes, phagocytes engulf and destroy invaders, and the complement system opsonizes, recruits immune cells, and can lyse bacteria. Lipid A is the lipid component of LPS in Gram-negative bacteria; when released, it acts as an endotoxin that triggers a strong inflammatory response rather than providing protection. Its release can contribute to fever and septic shock, not defense. So the thing that does not fit as a protective mechanism is the release of lipid A.