Which component is described as the body's first line of defense against foreign substances?

• A. Respiratory system
• B. Digestive system
• C. Lymphatic system
• D. Skin

Answer: (D)

The skin provides the body's first line of defense by forming a tough, intact outer barrier that prevents most foreign substances from entering. The epidermis contains keratin, making the skin durable and waterproof, while sweat and oils create a hostile, slightly acidic surface that inhibits microbial growth. Because this barrier is often the first obstacle pathogens encounter, it stops many invaders before they can reach internal tissues. Other defense systems—like the mucous membranes in the respiratory and digestive tracts and the lymphatic system—play crucial roles after a breach occurs, but they come into play later rather than serving as the initial shield.