- Describe the barriers that keep most pathogens out of the human body.
- Explain how the inflammatory response and nonspecific leukocytes help fight pathogens that enter the body.
- inflammatory response
- mucous membrane
The immune system protects the body from worms, germs, and other agents of harm. The immune system is like a medieval castle. The outside of the castle was protected by a moat and high stone walls. Inside the castle, soldiers were ready to fight off any invaders that managed to get through the outer defenses. Like a medieval castle, the immune system has a series of defenses. In fact, it has three lines of defense. Only pathogens that are able to get through all three lines of defense can harm the body.
The First Line of Defense
The body’s first line of defense consists of different types of barriers that keep most pathogens out of the body. Pathogens are disease-causing agents, such as bacteria and viruses. These and other types of pathogens are described in Figure below. Regardless of the type of pathogen, however, the first line of defense is always the same.
Types of pathogens that commonly cause human diseases include bacteria, viruses, fungi, and protozoa. Which type of pathogen causes the common cold? Which type causes athlete’s foot?
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 the body has. The outer layer of the skin is tough and very difficult for pathogens to penetrate.
Mucous membranes provide a mechanical barrier at body openings. They also line the respiratory, GI, urinary, and reproductive tracts. Mucous membranes secrete mucus, a slimy substance that traps pathogens. The membranes also have hair-like cilia. The cilia sweep mucus and pathogens toward body openings where they can be removed from the body. When you sneeze or cough, pathogens are removed from the nose and throat (see Figure below). Tears wash pathogens from the eyes, and urine flushes pathogens out of the urinary tract. You can watch the sweeping action of cilia at the following link: http://mcdb.colorado.edu/courses/2115/units/Other/mucus%20animation.swf.
A sneeze can expel many pathogens from the respiratory tract. That’s why you should always cover your mouth and nose and when you sneeze.
Chemical barriers destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, and saliva all contain enzymes that kill pathogens. Urine is too acidic for many pathogens, and semen contains zinc, which most pathogens cannot tolerate. In addition, stomach acid kills pathogens that enter the GI tract in food or water.
Biological barriers are living organisms that help protect the body. Millions of harmless bacteria live on the human skin. Many more live in the GI tract. The harmless bacteria use up food and space so harmful bacteria cannot grow.
The Second Line of Defense
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 body’s second line of defense.
The cut on your hand may become red, warm, and swollen. These are signs of an inflammatory response. This is the first reaction of the body to tissue damage or infection. As explained in Figure below, the response is triggered by chemicals called cytokines and histamines, which are released when tissue is injured or infected. The chemicals communicate with other cells and coordinate the inflammatory response. You can see an animation of the inflammatory response at this link: http://www.sumanasinc.com/webcontent/animations/content/inflammatory.html.
This drawing shows what happens during the inflammatory response. Why are changes in capillaries important for this response?
The inflammatory response is discussed at http://www.youtube.com/watch?v=FXSuEIMrPQk.
The chemicals that trigger an inflammatory response attract leukocytes to the site of injury or infection. Leukocytes are white blood cells. Their role is to fight infections and get rid of debris. Leukocytes may respond with either a nonspecific or a specific defense.
- A nonspecific defense is the same no matter what type of pathogen is involved. An example of a nonspecific defense is phagocytosis. This is the process in which leukocytes engulf and break down pathogens and debris. It is illustrated in Figure below. You can also see an animation of phagocytosis at this link: http://www.sp.uconn.edu/~terry/Common/phago053.html.
- A specific defense is tailored to a particular pathogen. Leukocytes involved in this type of defense are described in the next lesson.
In this image, leukocytes (white) are attacking pathogens (star-shaped).
A summary of the nonspecific defenses can be viewed at http://www.youtube.com/watch?v=O1N2rENXq_Y (16:20).
- Barriers that keep out pathogens are the body’s first line of defense. They include mechanical, chemical, and biological barriers.
- The second line of defense attacks pathogens that manage to enter the body. It includes the inflammatory response and phagocytosis by nonspecific leukocytes.
1. Identify three types of barriers in the body’s first line of defense. Give an example of each type of barrier.
2. What is the body’s second life of defense? When does it take effect?
3. Identify the roles of nonspecific leukocytes in the body’s second line of defense.
4. State how phagocytosis helps defend the body from pathogens.
5. What is a nonspecific defense?
6. Jera cut her finger. The next day, the skin around the cut had become red and warm. Why are these signs of infection?
7. Explain how the inflammatory response helps fight an infection.
Points to Consider
The body’s first and second lines of defense are the same regardless of the particular pathogen involved. The body’s third line of defense is different. It tailors the response to the specific pathogen.
- How do you think the immune system can identify specific pathogens?
- How might a specific defense be different from a nonspecific defense? What mechanisms might be involved?