The first line of host defense is the barrier presented to the external environment. Which of the following statement(s) is/are true concerning host barriers?

b. Within the respiratory tract, ciliary function serves to extrude microorganisms trapped within the mucus secretion layer

c. The low pH within the stomach markedly decreases bacterial content of the upper gastrointestinal tract

d. Gut peristalsis serves to prevent microbial adherence and invasion

The skin, mucus membranes, and epithelial layers of various organs of the body constitute effective physical barriers against microbial invasion. In certain portions of the body, these barriers have developed ancillary adaptations to increase the effectiveness of the barrier functions. Skin structures such as sebaceous glands secrete chemical compounds that serve to maintain a relatively low pH, providing effective bacterial stasis. Mucus secretion by specialized glands within the bronchi and gut provide a mucus layer that represents a physical and chemical barrier to microbial invasion. Within the respiratory tract, ciliary function serves to extrude microorganisms trapped within this mucus layer. In the alimentary track, the very low pH within the stomach and gut peristalsis both serve to prevent microbial adherence and invasion.