Which statement correctly identifies the body's first line of defense?

Which statement correctly identifies the body's first line of defense?

• A. Respiratory system traps particles using hairs and moist mucous membranes.
• B. Skin forms the body's first line of defense against foreign substances.
• C. Digestive enzymes destroy invaders swallowed.
• D. Nervous system provides barrier against pathogens.

Answer: (B)

The main idea is that the first line of defense consists of physical barriers that block pathogens from entering the body, with intact skin serving as the primary barrier. The skin’s tough outer layer and its secretions create an environment that’s hostile to microbes and prevents entry. This makes the statement about skin forming the body's first line of defense the best fit because it directly identifies the main exterior barrier that stops pathogens at the point of entry. The other options describe protective measures, but not the primary exterior barrier. Trapping particles in the respiratory tract relies on mucous membranes, which are part of barrier defenses but do not single-handedly define the first line as strongly as intact skin. Digestive enzymes and stomach acid act as chemical barriers once something is swallowed, which is a later line of defense relative to the initial exterior barrier. The nervous system does not serve as a physical barrier to pathogens.

The main idea is that the first line of defense consists of physical barriers that block pathogens from entering the body, with intact skin serving as the primary barrier. The skin’s tough outer layer and its secretions create an environment that’s hostile to microbes and prevents entry. This makes the statement about skin forming the body's first line of defense the best fit because it directly identifies the main exterior barrier that stops pathogens at the point of entry.

The other options describe protective measures, but not the primary exterior barrier. Trapping particles in the respiratory tract relies on mucous membranes, which are part of barrier defenses but do not single-handedly define the first line as strongly as intact skin. Digestive enzymes and stomach acid act as chemical barriers once something is swallowed, which is a later line of defense relative to the initial exterior barrier. The nervous system does not serve as a physical barrier to pathogens.