Superficial Defense

Abstract

Before birth, the developing embryo is defended against infection with microorganisms except those capable of vertical (transovarial or transplacental) infections like Rubella virus, the retroviruses, or Treponema pallidum by its mother’s host-defense mechanisms throughout its intrauterine life. But emerging from the uterus, the infant comes into direct contact with microorganisms. Thus, the role of skin and mucous membranes covering the body surface as the host-defense mechanisms should be clearly considered as they are the first point of contact with the external world of microorganisms. Furthermore, there are deeply internalized tubes and cavities connected to the body surface, e.g., naso-pharynx, trachea, bronchi, lung alveoli; oral pharyngial cavities and the esophago-gastrointestinal tracts of digestive organs; urethra, and the vagina and uterus in female and prostate and seminal duct systems in male and auditory canals which reach to the tympanic membrane. In a strict sense, the inner surfaces of all of these ducts and cavities are, also, boundaries between the inner and outer world of the body, where the microorganisms make their first contact with the host. However, not only natural mechanical barriers, but also acquired resistance mechanisms which will be mentioned later, function on the surfaces of most internalized tubes and cavities, in contrast with the skin and the corneal surfaces. These mechanical barriers will be described as follows.