Which of the following is considered a physical barrier in the immune system?

• A. Mucus
• B. Stomach acid
• C. Tears
• D. Skin

Answer: (D)

Verified step by step guidance

1. Understand the concept of physical barriers in the immune system: Physical barriers are the first line of defense in the immune system, preventing pathogens from entering the body. These barriers are structural or mechanical features that block or trap harmful microorganisms.
2. Review the options provided: Mucus, stomach acid, tears, and skin. Determine which of these options fits the definition of a physical barrier.
3. Analyze each option: Mucus, tears, and stomach acid are chemical barriers because they contain substances that trap or destroy pathogens. For example, mucus traps pathogens, tears contain lysozymes that break down bacterial cell walls, and stomach acid destroys pathogens with its low pH.
4. Focus on skin: Skin is a physical barrier because it acts as a tough, protective layer that prevents pathogens from entering the body. It is composed of tightly packed cells and keratin, making it impermeable to most microorganisms.
5. Conclude that skin is the correct answer: Based on the definition of physical barriers and the analysis of the options, skin is the structural feature that serves as a physical barrier in the immune system.