Physical and Chemical Barriers: The First Line of Defense

The epithelia layer that covers the outside of our body and lines lumens, cavities and spaces is the first barrier or defense that must be overcome by a microbial infection. The skin, digestive tract lining, and respiratory system lining all show age-related changes that affect their structural integrity. Examples include thinning of the skin, atrophy of the vaginal wall, and loss of cilia in the respiratory tract, all of which decrease the effectiveness of many “physical” barriers in the elderly. Some of the chemical barriers include salivary secretions, mucus secretions, tears, gastric secretion, and vaginal secretions. These often have pH levels that inhibit growth of bacteria and also contain antibodies and anti-bacterial agents. Atrophy of these glandular elements increases the vulnerability of older individuals to infections. However, with proper hygiene, chemical barrier defense mechanisms continue to function adequately throughout the life span despite progressive decline of glandular function.

Several additional, non-specific forces are also involved in the first line of defense. Under the lining of all tissues are monocyte-derived macrophages that engulf viruses and bacteria on contact. If a virus should escape such phagocytosis and infect a tissue cell, the cell produces protective chemicals (such as interferon) that inhibit viral replication. Should all these barriers be breached and the bacterium or virus enters the blood stream, the blood contains protective chemicals (the complement system) that can prevent multiplication of the infectious organism. The complement system is a complex of plasma enzymes normally involved in antigen-antibody reactions, but complement can initiate bacterial or viral lysis in the absence of antibodies. A final non-specific response is the induction of fever, which is protective in some instances because some microbes cannot tolerate a rise in blood temperature. The pathogenesis and importance of fever remains unclear. Apparently, when bacteria are digested by macrophages, bacterial toxins (endotoxins) activate the release of chemicals such as interleukins, which enters the brain hypothalamus and causes a resetting of hypothalamic temperature receptors.

In brief, the first line of defense involves a widespread array of physical, secretory, cellular, and plasma chemical factors that protect the body from any and all types of infectious agents. In many respects it is the most important of our body defenses. It prevents entry of most of the millions of pathogens the body encounters every minute of our life. The simple observation that the inflammatory or immune responses that are described below are activated relatively infrequently attests to the operational adequacy of the first line of defense throughout our life span.