What are the structures in the nose and respiratory tract that help fight off pathogens?

The correct choice highlights the critical role of mucus and cilia in the respiratory system's defense against pathogens.

Mucus serves as a sticky barrier that traps dust, bacteria, viruses, and other harmful particles entering through the nasal passages. When pathogens adhere to the mucus, they are less likely to reach the lungs and cause infection. Cilia, which are tiny hair-like structures lining the respiratory tract, beat in a coordinated manner to propel the mucus, along with the trapped particles, out of the respiratory system. This action effectively clears the airways, further reducing the likelihood of respiratory infections.

While white blood cells, nasal hairs, and antibodies are all important components of the immune response, they operate at different stages of pathogen defense. White blood cells are primarily involved after the body has been breached by pathogens, whereas nasal hairs provide an initial filtration mechanism but are not as crucial as mucus and cilia in actively removing particles from the respiratory tract. Antibodies are produced in response to infections and are not a direct mechanism of trapping or clearing pathogens from the respiratory tract. Therefore, mucus and cilia are fundamental first-line defenses against inhaled pathogens.