Which structure protects against bacteria entering oral and nasal cavities?

• A. Spleen
• B. Lymph
• C. Tonsils
• D. Alloimmunity

Answer: (C)

Tonsils act as a frontline defense at the entry points of the mouth and nose. They form Waldeyer's ring, a ring of lymphoid tissue in the pharynx that includes the palatine tonsils, pharyngeal (adenoids), and lingual tonsils. These tissues sample incoming air and swallowed secretions, trap bacteria, and mount immune responses—helping to neutralize pathogens before they can invade deeper tissues. They also contribute to mucosal antibody production, especially secretory IgA, which helps protect the lining of the mouth and throat.

The spleen is involved in filtering blood and mounting systemic responses, not guarding the mucosal entry points. Lymph refers to circulating immune tissue in general rather than a specific protective structure at the entry, and alloimmunity is an immune reaction to non-self antigens from another individual, not a local defense at oral and nasal surfaces.