1) MECHANICAL (PHYSICAL) BARRIERS: tight junctions between the epithelial cells prevent the penetrations of pathogens between the cells to underlying tissues. In addition there is a flow of air and fluid over epithelial surface,s which oxgenates and flushes the surfaces, preventing anaerobic bacterial growth and transient adhesion. On ciliated epithelial surfaces, such as those of the respiratory tract, the formation of a layer of mucus that is kept in continual movement by the beating cilia inhibits colonization and invasion by microorganisms.
2) CHEMICAL BARRIERS: the epithelium produes a variety of chemical substances that interefere with the adherence of microoragnisms to epithelium and with their replication. The skin helps to create an acid environment inhibitory to the growth of many bacteria. Lysozyme, an enzyme that inhibitis cell-wall formation in bacteria, is secreted in tears, saliva, and sweat. The stomach produces strong hydrochloric acid, creating a highly acidic and formidable environment, which when combined with the stomach enzyme pepsin (and acid protease) poses one of the most inhospitable environments for microbial peptides secreted by all the protective epithelia.
3) MICROBIOLOGICAL BARRIERS: a flora of non-pathogenic commensal microorganisms colonizes many epithelial surfaces and provides an additional barrier to infection. These microorganisms compete with pathogenic microbes for space and nutrients, and sometimes produce antibacterial protiens that further inhibit attachment to epithelium.