A First Line of Defense Includes the Skin and Certain External and Internal Body Fluids
The physical nonimmune defense barriers include external body surfaces such as skin and internal body surfaces such as the gastrointestinal (GI), reproductive, respiratory, and the urogenital tracts.
The skin plays a major role in preventing the entry of organisms through a variety of nonimmunological means, including the secretion of sebum from sebaceous glands, which maintains a low pH, and secretion of enzymes that are not conducive for the invading pathogens. Periodic natural desquamation of the skin also results in sloughing off any invading pathogens. Nonpathogenic bacteria also occupy skin surfaces, thereby preventing the adherence of pathogenic organisms to their target cells, which is prerequisite for entry into the body. Any changes in the skin, such as cuts, burns, and dry or very humid skin, will result in the entry of microbes. In addition to the nonimmunological mechanisms, skin is also rich in dendritic cells (Langerhans cells) and γ-δ T cells that contribute to warding off invading pathogens. The natural flushing action of urine and milk assist in elimination of infectious antigens, as evidenced by the infectious conditions that result from stasis of urine or milk.Many body fluids are inhospitable to invading pathogens. For example, mucus in the mucosal tissues (respiratory, urogenital, and Gl tracts), saliva, tears, gastric juices, and urine are rich in enzymes (e.g., lysozymes) and are low in pH. As with the skin, the Gl tract is covered with Iionpathogenic bacteria, which prevent the adhesion of pathogenic bacteria to their target cells. Furthermore, resident normal bacterial florae in gastric tissues secrete butyric or lactic acids, which not only maintain a low pH in gastric fluids, but also are bacteriostatic to other microbes. Vaginal epithelium is rich in glycogen and promotes the growth of Lactobacillusi which secretes lactic acids. In the respiratory tract the antigen load is decreased by a variety of mechanisms, including the turbulence created when air is inhaled due to the anatomical construction of the lower respiratory tract, which narrows and branches. Microorganisms in inhaled air are carried by this turbulence and are forced onto the walls of the respiratory tract» which are rich in sticky mucus and bactericidal lysozymes. The ciliary action of the respiratory tract also eliminates antigens effectively.