9.3 Natural Defenses Against Infection
Open Resources for Nursing (Open RN)
There are two basic ways the body defends against pathogens: nonspecific innate immunity and specific adaptive immunity.
Nonspecific Innate Immunity
Nonspecific innate immunity is a system of defenses in the body that targets invading pathogens in a nonspecific manner. It is called “innate” because it is present from the moment we are born. Nonspecific innate immunity includes physical defenses, chemical defenses, and cellular defenses.[1]
Physical Defenses
Physical defenses are the body’s most basic form of defense against infection. They include physical barriers to microbes, such as skin and mucous membranes, as well as mechanical defenses that physically remove microbes and debris from areas of the body where they might cause harm or infection. In addition, a person’s microbiome provides physical protection against disease as normal flora compete with pathogens for nutrients and cellular-binding sites.[2]
Skin
One of the body’s most important physical barriers is the skin barrier, which is composed of three layers of closely packed cells. See Figure 9.5[3] for an illustration of the layers of skin. The topmost layer of skin called the epidermis consists of cells that are packed with keratin. Keratin makes the skin’s surface mechanically tough and resistant to degradation by bacteria. Infections can occur when the skin barrier is broken, allowing the entry of opportunistic pathogens that infect the skin tissue surrounding the wound and possibly spread to deeper tissues.[4]
Mucus Membranes
The mucous membranes lining the nose, mouth, lungs, and urinary and digestive tracts provide another nonspecific barrier against potential pathogens. Mucous membranes consist of a layer of epithelial cells bound by tight junctions. The epithelial cells secrete a moist, sticky substance called mucous. Mucous covers and protects the fragile cell layers beneath it and also traps debris, including microbes. Mucus secretions also contain antimicrobial peptides.[5]
In many regions of the body, mechanical actions flush mucus (along with trapped or dead microbes) out of the body or away from potential sites of infection. For example, in the respiratory system, inhalation can bring microbes, dust, mold spores, and other small airborne debris into the body. This debris becomes trapped in the mucus lining the respiratory tract. The epithelial cells lining the upper parts of the respiratory tract have hair-like appendages known as cilia. Movement of the cilia propels debris-laden mucus out and away from the lungs. The expelled mucus is then swallowed and destroyed in the stomach, coughed up, or sneezed out. This system of removal is often called the mucociliary escalator. Disruption of the mucociliary escalator by the damaging effects of smoking can lead to increased colonization of bacteria in the lower respiratory tract and frequent infections, which highlights the importance of this physical b