Question

which part of the immune system is a physical barrier?
skin
tears
mucus
stomach acid

Explanation:

To determine the physical barrier of the immune system, we analyze each option:

• Tears: Contain enzymes (like lysozyme) and are part of the chemical/enzymatic defense, not a physical barrier.
• Mucus: Traps pathogens but is more of a mucous membrane - associated defense (can be considered a physical - chemical barrier, but not the primary physical barrier like skin).
• Stomach acid: Is a chemical defense (low pH kills pathogens).
• Skin: It is the outermost layer of the body and acts as a physical barrier, preventing the entry of most pathogens by forming a continuous layer.

Answer:

A. skin