What structure lines the nose, throat, and respiratory tract to protect against infection?

The structure that lines the nose, throat, and respiratory tract and serves to protect against infection is the mucous membranes. These membranes are specialized tissues that produce mucus, a thick fluid that traps pathogens, dust, and other foreign particles. By doing so, they function as a barrier, preventing these potentially harmful substances from entering the body and causing infections.

Mucous membranes play a crucial role in the immune defense of the respiratory system. The mucus contains various antimicrobial substances, including enzymes and antibodies, that actively neutralize bacteria and viruses. Additionally, the presence of cilia—tiny hair-like structures on the surface of these membranes—helps move mucus out of the respiratory tract, further aiding in the removal of trapped pathogens and debris.

While skin provides a protective barrier to external threats, it does not line the internal structures like the nose and throat. Saliva, although it has some protective qualities in the mouth, is not the primary lining found in the respiratory tract. Therefore, the mucous membranes are the most accurate answer when considering what specifically lines and protects the nasal and respiratory passages from infection.