The immune system is the body's second line of defense against infection by pathogens.

• A. First line of defense
• B. Third line of defense
• C. All of the above
• D. Second line of defense

Answer: (D)

The idea being tested is how the body defends itself in stages called lines of defense. The first line are physical and chemical barriers, like intact skin and mucous membranes, which block most pathogens from entering. If a pathogen gets past those barriers, the second line, which is the innate immune response, kicks in. This includes nonspecific defenses such as inflammation, fever, and immune cells like neutrophils and macrophages that engulf and destroy invaders. These responses happen quickly and do not require prior exposure to the pathogen. The third line of defense is adaptive immunity, involving specific T and B cells and antibodies that target particular pathogens and remember them for faster responses in the future. Therefore, saying the immune system is the body's second line of defense aligns with this framework, since the innate immune response provides the immediate, non-specific defense after barrier breach.