Immune System: Nonspecific Defenses

The immune system protects the body from ”germs” and other harmful substances. The immune system is like a medieval castle. The outside of a medieval castle was protected by a moat and high stone walls. Inside the castle, soldiers were ready to defend the castle against any invaders that got through the outer defenses. Like a medieval castle, the immune system has a series of defenses. Only pathogens that are able to get through all the defenses can cause harm to the body.

First Line of Defense

The immune system has three lines of defense. The first line of defense includes a variety of barriers against pathogens that keep most pathogens out of the body. Pathogens are disease-causing agents, such as bacteria and viruses. Defenses in the first line are the same regardless of the type of pathogen. This is why they are called nonspecific defenses. Several types of pathogens that are common causes of human disease can be seen in the Figure 1.

Figure 1: Common Human Pathogens.

Mechanical Barriers

Mechanical barriers physically block pathogens from entering the body. The skin is the most important mechanical barrier. In fact, it is the single most important defense of the body against pathogens. It forms a physical barrier between the body and the outside world. The outer layer of the skin is a tough, nearly water-proof coating that is very difficult for pathogens to penetrate.

At body openings, such as the mouth and nose, the body has a different mechanical barrier. Instead of skin, mucous membranes line these and other organs that are exposed to the outside environment. They include the organs of the respiratory, gastrointestinal, and urinary tracts. Mucous membranes secrete mucus, a slimy substance that coats the membranes and traps pathogens. Mucous membranes also have cilia, which are tiny projections that have wavelike motions. The movements of cilia sweep mucus and trapped pathogens toward body openings to be removed from the body.

Pathogens are removed from the respiratory tract when you sneeze or cough. In addition, tears wash pathogens from the eyes, and urine flushes pathogens out of the urinary tract.

Chemical Barriers

Chemical barriers are proteins that destroy pathogens at the body’s surface. The skin and mucous membranes secrete proteins that kill many of the pathogens with which they come into contact. For example, enzymes called lysozymes—which are found in sweat, mucus, tears, and saliva—kill pathogens by breaking open their cell walls. Urine and vaginal secretions are too acidic for many pathogens, and semen contains zinc, which most pathogens cannot tolerate. Hydrochloric acid secreted by mucous membranes lining the stomach kills pathogens that enter the stomach in food or water.

Biological Barriers

Biological barriers involve living organisms that compete with pathogens. Human skin is covered by millions of bacteria. Millions more colonize the gastrointestinal, urinary, and genital tracts. Most of these bacteria are helpful or at least not harmful. They are important in defense because they help prevent harmful bacteria from becoming established in or on the body. They do this by competing with harmful bacterial for food and space. Helpful bacteria may also change pH or other factors and make conditions less suitable for harmful bacteria.

Second Line of Defenses

If you have a cut on your hand, the break in the skin provides a way for pathogens to enter your body. Assume bacteria enter through the cut and infect the wound. These bacteria would then encounter the second line of defense.

Inflammatory Response

The cut on your hand is likely to become red, warm, swollen, and painful. These are all signs that an inflammatory response has occurred. An inflammatory response is a complex biological reaction to tissue damage. It is one of the first responses of the immune system to infection or injury. Inflammation is triggered by chemicals called cytokines and histamines, which are released when tissues are damaged.

• Cytokines are chemical signals used to communicate between cells.

• Histamines are chemicals that cause inflammation and allergies.

The cytokines and histamines released when tissue is damaged cause many changes in the damaged tissue. The changes help remove the cause of the damage and start the healing process. For example, the chemicals cause local blood vessels to dilate, which increases blood flow to the area. They also cause other changes in blood vessels that allow blood components to leak into the damaged tissue.

White Blood Cells

Another role of cytokines is to attract white blood cells, or leukocytes, to the site of inflammation. Leukocytes are immune system cells that are specialized to fight infections. They are the primary cells of the immune system and found throughout the body. The general function of leukocytes is to identify and eliminate pathogens, debris, and abnormal body cells. Figure 2 shows several different types of leukocytes. Each type plays a different role in the removal of pathogens and other unwanted substances from the body.

Figure 2: Types of Leukocytes.

Some leukocytes are nonspecific and respond in the same way to most pathogens. Nonspecific leukocytes include monocytes, macrophages, neutrophils, eosinophils, and basophils. These leukocytes are part of the second line of defense. A magnified image of an actual macrophage is shown in Figure 3.

Figure 3: Magnified image of a macrophage.

Monocytes, macrophages, and neutrophils destroy pathogens in the blood and tissues by phagocytosis. Phagocytosis is the process of engulfing and breaking down pathogens and other unwanted substances. Phagocytosis of a pathogen by a macrophage is illustrated in Figure 4. Once a pathogen has been engulfed, it is broken down within the macrophage. Macrophages are found in tissues, and monocytes and neutrophils are found in the blood.

Figure 4: Phagocytosis by a macrophage.

Both monocytes and neutrophils migrate through the bloodstream to sites of inflammation. Neutrophils are the most common leukocytes and usually the first leukocytes to arrive at the scene of infection. Neutrophils and dead pathogens are the main components of pus.

In addition to phagocytosis, both monocytes and phagocytes produce chemicals such as cytokines that cause inflammation and fever. A fever is a higher-than-normal body temperature that may help fight infection. Monocytes or macrophages may also trigger the third line of defense.

Eosinophils and basophils are responsible for allergies. Eosinophils also help fight infections by combating parasites such as protozoa. Basophils release cytokines, histamines, and other chemicals that contribute to inflammation as well as allergies.

Lymphocytes are different from these nonspecific leukocytes. Lymphocytes launch an attack that is tailored to a particular pathogen. For example, some lymphocytes attack only herpes viruses, others only flu viruses. This is called a specific defense.