Two physical barriers include which structures?

• A. Skin and mucous membrane
• B. Antibodies and complement
• C. Lymph nodes and spleen
• D. Interferons and fever

Answer: (A)

The first line of defense consists of physical barriers that prevent microbes from entering the body. The skin provides a continuous, dry, keratinized surface that resists penetration, and its outer layers regularly shed to remove trapped microbes. Mucous membranes line openings to the outside (such as the respiratory, digestive, urinary, and reproductive tracts) and secrete mucus to trap microbes; in the airways, cilia and the mucociliary escalator move that mucus out of the body, helping wash pathogens away. Together, these barriers block entry and help keep invading organisms from gaining a foothold.

Antibodies and complement, lymph nodes and spleen, and interferons with fever are important parts of the immune response once a barrier is breached, but they are not physical barriers themselves.