In addition, the dead, keratinized cells that make up the surface of the skin are continuously being sloughed off so that microbes that do colonize these cells are constantly being removed. Hair follicles and sweat glands produce lysozyme and toxic lipids that can kill bacteria. Epithelial cells also produce defensins and cathelicidins to kill microbes. Beneath the epidermis of the skin are Langerhans' cells - immature dendritic cells - that phagocytose and kill microbes, carry them to nearby lymph nodes, and present antigens of these microbes to T-lymphocytes to begin adaptive immune responses against them.
Finally, intraepithelial T-lymphocytes and B-1 lymphocytes are associated with the epidermis and the mucosal epithelium. These cells recognize microbes common to the epidermis and mucous membranes and start immediate adaptive immune responses against these commonly encountered microbes.
Mucous membranes line body cavities that open to the exterior, such as the respiratory tract, the gastrointestinal tract, and the genitourinary tract. Mucous membranes are composed of an epithelial layer that secretes mucus, and a connective tissue layer. The mucus is a physical barrier that traps microbes.
Mucus also contains lysozyme to degrade bacterial peptidoglycan, an antibody called secretory IgA that prevents microbes from attaching to mucosal cells and traps them in the mucous, lactoferrin to bind iron and keep it from from being used by microbes, and lactoperoxidase to generate toxic superoxide radicals that kill microbes.
Resident normal microbiota of the mucosa also inhibits potentially harmful microbes. In addition, the mucous membrane, like the skin, is constantly sloughing cells to remove microbes that have attached to the mucous membranes.
Beneath the mucosal membrane is mucosa-associated lymphoid tissue MALT that contains Langerhans' cells - immature dendritic cells - that phagocytose and kill microbes, carry them to nearby lymph nodes, and present antigens of these microbes to T-lymphocytes to begin adaptive immune responses against them. Intraepithelial T-lymphocytes and B-1 lymphocytes are associated with the epidermis and the mucosal epithelium.
Bony encasements, such as the skull and the thoracic cage, protect vital organs from injury and entry of microbes.
Free learning from The Open University. Featured content. Free courses. All content. Course content. Session 4. About this free course 12 hours study. Level 1: Introductory. Course rewards. Free statement of participation on completion of these courses. Create your free OpenLearn profile. Course content Course content. Also, the normal flow of urine washes out microorganisms that enter the urinary tract. The immune system uses white blood cells White blood cells The immune system is designed to defend the body against foreign or dangerous invaders.
See also Overview of Infectious Disease Overview of Infectious Disease Microorganisms are tiny living creatures, such as bacteria and viruses. It serves many important functions, including Protecting the body against trauma Regulating body temperature Maintaining water and electrolyte balance Sensing Mucous membranes, such as the lining of the mouth, nose, and eyelids, are also effective barriers.
Typically, mucous membranes are coated with secretions that fight microorganisms. For example, the mucous membranes of the eyes are bathed in tears, which contain an enzyme called lysozyme that attacks bacteria and helps protect the eyes from infection. The airways Overview of the Respiratory System To sustain life, the body must produce sufficient energy. Energy is produced by burning molecules in food, which is done by the process of oxidation whereby food molecules are combined with The walls of the passages in the nose and airways are coated with mucus.
Microorganisms in the air become stuck to the mucus, which is coughed up or blown out of the nose. Mucus removal is aided by the coordinated beating of tiny hairlike projections cilia that line the airways.
The cilia sweep the mucus up the airways, away from the lungs. The digestive tract Overview of the Digestive System The digestive system, which extends from the mouth to the anus, is responsible for receiving food, breaking it down into nutrients a process called digestion , absorbing the nutrients into The acini produce digestive enzymes See also Overview of the Liver and Gallbladder These substances can kill bacteria or prevent them from multiplying.
The contractions of the intestine peristalsis, which moves contents of the bowel through the digestive tract , and the normal shedding of cells lining the intestine help remove harmful microorganisms. The urinary tract Overview of the Urinary Tract Normally, a person has two kidneys. The rest of the urinary tract consists of the following: Two ureters the tubes connecting each kidney to the bladder The bladder an expandable muscular The bladder is protected by the urethra Urethra The urethra is a tube that drains urine from the bladder out of the body.
In men, the urethra is about 8 inches 20 centimeters long, ending at the tip of the penis. In women, the urethra is In males, the urethra is long enough that bacteria are seldom able to pass through it to reach the bladder, unless the bacteria are unintentionally placed there by catheters or surgical instruments.
Infections can occur when the skin barrier is compromised or broken. A wound can serve as a point of entry for opportunistic pathogens, which can infect the skin tissue surrounding the wound and possibly spread to deeper tissues. Mike, a gardener from southern California, recently noticed a small red bump on his left forearm. Initially, he did not think much of it, but soon it grew larger and then ulcerated opened up , becoming a painful lesion that extended across a large part of his forearm Figure 3.
He went to an urgent care facility, where a physician asked about his occupation. Figure 3. Under most conditions, fungi cannot produce skin infections in healthy individuals. Fungi grow filaments known as hyphae, which are not particularly invasive and can be easily kept at bay by the physical barriers of the skin and mucous membranes. Once it breaches the skin barrier, S. Compounding matters, other pathogens may enter the infected tissue, causing secondary bacterial infections.
His lesions eventually healed, and Mike returned to work with a new appreciation for gloves and protective clothing. The mucous membrane s 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 mucus , which covers and protects the more fragile cell layers beneath it and traps debris and particulate matter, including microbes.
Mucus secretions also contain antimicrobial peptides. Figure 4. This scanning electron micrograph shows ciliated and nonciliated epithelial cells from the human trachea. The mucociliary escalator pushes mucus away from the lungs, along with any debris or microorganisms that may be trapped in the sticky mucus, and the mucus moves up to the esophagus where it can be removed by swallowing.
In many regions of the body, mechanical actions serve to 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, a layer known as the mucociliary blanket.
The epithelial cells lining the upper parts of the respiratory tract are called ciliated epithelial cells because they 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, or coughed up, or sneezed out Figure 4.
This system of removal is often called the mucociliary escalator. The mucociliary escalator is such an effective barrier to microbes that the lungs, the lowermost and most sensitive portion of the respiratory tract, were long considered to be a sterile environment in healthy individuals.
Only recently has research suggested that healthy lungs may have a small normal microbiota. Disruption of the mucociliary escalator by the damaging effects of smoking or diseases such as cystic fibrosis can lead to increased colonization of bacteria in the lower respiratory tract and frequent infections, which highlights the importance of this physical barrier to host defenses. Like the respiratory tract, the digestive tract is a portal of entry through which microbes enter the body, and the mucous membranes lining the digestive tract provide a nonspecific physical barrier against ingested microbes.
The intestinal tract is lined with epithelial cells, interspersed with mucus-secreting goblet cells Figure 5.
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