Which statement regarding the body's first line of defense is most accurate?

Which statement regarding the body's first line of defense is most accurate?

• A. Fever
• B. Antibodies
• C. White blood cells
• D. Skin and mucous membranes

Answer: (D)

The first line of defense consists of barriers on the body's surfaces that prevent pathogens from entering. The skin is the primary physical barrier—its tough, intact epidermis blocks microbes from getting inside, and secretions like sweat and oils help inhibit microbial growth. Mucous membranes line the airways, digestive and urogenital tracts; they trap particles in mucus and rely on cilia to move invaders out, while mucus itself, along with enzymes like lysozyme in tears and saliva, provides chemical protection. Stomach acid creates a harsh environment that can kill many ingested pathogens, and the normal microbial flora on skin and mucous surfaces competes with invaders, further reducing infection risk. Fever, antibodies, and white blood cells are part of later defense layers: fever is a systemic response that helps limit pathogen growth, antibodies are produced during adaptive immunity to target specific pathogens, and white blood cells include innate responders that act after barriers are breached.

The first line of defense consists of barriers on the body's surfaces that prevent pathogens from entering. The skin is the primary physical barrier—its tough, intact epidermis blocks microbes from getting inside, and secretions like sweat and oils help inhibit microbial growth. Mucous membranes line the airways, digestive and urogenital tracts; they trap particles in mucus and rely on cilia to move invaders out, while mucus itself, along with enzymes like lysozyme in tears and saliva, provides chemical protection. Stomach acid creates a harsh environment that can kill many ingested pathogens, and the normal microbial flora on skin and mucous surfaces competes with invaders, further reducing infection risk. Fever, antibodies, and white blood cells are part of later defense layers: fever is a systemic response that helps limit pathogen growth, antibodies are produced during adaptive immunity to target specific pathogens, and white blood cells include innate responders that act after barriers are breached.