Which body system forms the body's first line of defense against foreign substances?

Which body system forms the body's first line of defense against foreign substances?

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

Answer: (A)

The first line of defense is a physical barrier that blocks pathogens from entering the body. The skin serves this role as the outermost protective layer—its thick, keratinized epidermis keeps microbes from crossing into deeper tissues. Sweat and sebum create a chemical barrier with low pH and antimicrobial substances that inhibit many microbes, and the skin’s normal flora helps crowd out potential pathogens. Immune surveillance in the skin, including Langerhans cells, can detect invaders and alert the immune system to respond quickly. Other body systems contribute to defense once a breach occurs or pathogens are inhaled or ingested, such as mucous membranes and cilia in the respiratory tract or stomach acid in the digestive tract, and the lymphatic system mobilizes immune responses. But the skin is the body's first protective barrier.

The first line of defense is a physical barrier that blocks pathogens from entering the body. The skin serves this role as the outermost protective layer—its thick, keratinized epidermis keeps microbes from crossing into deeper tissues. Sweat and sebum create a chemical barrier with low pH and antimicrobial substances that inhibit many microbes, and the skin’s normal flora helps crowd out potential pathogens. Immune surveillance in the skin, including Langerhans cells, can detect invaders and alert the immune system to respond quickly.

Other body systems contribute to defense once a breach occurs or pathogens are inhaled or ingested, such as mucous membranes and cilia in the respiratory tract or stomach acid in the digestive tract, and the lymphatic system mobilizes immune responses. But the skin is the body's first protective barrier.