First Line of Defense
Your body’s first line of defense is like a castle’s moat and walls. It keeps most pathogens out of your body. The first line of defense includes different types of barriers.
Skin and Mucous Membranes
The skin is a very important barrier to pathogens. The skin is the body’s largest organ. In adults, it covers an area of 1.5 to 2 square meters (about 16 to 22 square feet)! The skin is also the body’s single most important defense. It forms a physical barrier between the body and the outside world. As shown in Figure below, the skin has several layers. The outer layer is tough and waterproof. It is very difficult for pathogens to get through this layer of skin.
Figure 21.16
This drawing shows that the skin has many layers. The outer layer is so tough that it keeps out most pathogens.
The mouth and nose are not lined with skin. Instead, they are lined with mucous membranes. Other organs that are exposed to the outside world, including the lungs and stomach, are also lined with mucous membranes. Mucous membranes are not tough like skin. However, they have other defenses.
One defense of mucous membranes is the mucus they secrete. Mucus is a sticky, moist substance that coats mucous membranes. Most pathogens get stuck in the mucus before they can do harm to the body. Many mucous membranes also have cilia. Cilia in the lungs are shown in Figure below. Cilia are like tiny fingers. They move in waves and sweep mucus and trapped pathogens toward body openings. When you clear your throat or blow your nose, you rid your body of the mucus and pathogens.
Figure 21.17
This is what the cilia lining the lungs look like when they are magnified. Their movements constantly sweep mucus and pathogens out of the lungs. Do they remind you of brushes?
Chemicals
Most body secretions contain chemicals that kill pathogens. For example, mucus, sweat, tears, and saliva contain enzymes that kill pathogens. The enzymes are called lysozymes. They break down the cell walls of bacteria. The stomach secretes a very strong acid, called hydrochloric acid. This acid kills most pathogens that enter the stomach in food or water. Urine is also acidic, so few pathogens can grow in it.
Helpful Bacteria
You are not aware of them, but your skin is covered by millions (or more!) of bacteria. Millions more live inside your body. From the Food and Digestive System chapter, you know that many bacteria live inside your large intestine. Most of these bacteria help defend your body from pathogens. How do they do it? They compete with harmful bacteria for food and space. This prevents the harmful bacteria from multiplying and making you sick.
Second Line of Defense
The little girl in Figure below has a scraped knee. A scrape is a break in the skin that may let pathogens enter the body. If bacteria enter through the scrape, they could cause an infection. These bacteria would then face the body’s second line of defense.
Figure 21.18
This little girl just got her first scraped knee. It doesnt seem to hurt, but the break in her skin could let pathogens enter her body. Thats why scrapes should be kept clean and protected until they heal.
Inflammation
If bacteria enter the skin through a scrape, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. Inflammation is triggered by chemicals that are released when skin or other tissues are damaged. The chemicals cause nearby blood vessels to dilate, or expand. This increases blood flow to the damaged area. The chemicals also attract white blood cells to the wound and cause them to leak out of blood vessels into the damaged tissue. You can watch a video animation of this process at http://biology-animations.blogspot.com/search/label/inflammatory%20response%20animation.
White Blood Cells
After white blood cells leave a blood vessel at the site of inflammation, they start “eating” pathogens. From the Cardiovascular System chapter, you know that white blood cells are one type of cells that make up the blood. The main role of white blood cells is to fight pathogens in the body. There are actually several different kinds of white blood cells. Some white blood cells are very specialized. They attack only certain pathogens. You will read about these white blood cells in Lesson 4.
Other white blood cells attack any pathogens they find. These white blood cells travel to sites of inflammation. They are called phagocytes, which means “eating cells.” In addition to pathogens, phagocytes “eat” dead cells and other debris. They engulf the pathogens or debris and destroy them. This process is called phagocytosis. How phagocytosis occurs is shown in Figure below. You can watch a video of an actual phagocyte gobbling up and destroying a pathogen at http://sciencevideos.wordpress.com/category/phagocytosis/.
Figure 21.19
These drawings show phagocytosis. In this process, a phagocyte engulfs and breaks down a pathogen.
White blood cells also produce chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 98.6° F (37° C). Most bacteria and viruses that infect people multiply fastest at this temperature. When the temperature is higher, the pathogens cannot multiply as fast. A fever also triggers the immune system to make more white blood cells. In these ways, a fever helps the body fight infection.
Lesson Summary
Your body’s first line of defense includes the skin and other barriers that keep pathogens out of your body.
If pathogens enter your body, inflammation occurs, and phagocytes come to the body’s defense.
Review Questions
How does your skin protect you from pathogens?
What is mucus?
Define inflammation.
What are phagocytes?
What is a fever?
Explain how cilia help rid your body of pathogens.
How do helpful bacteria defend your body?
How does inflammation help fight pathogens?
Why is phagocytosis called a general body defense?
A fever is a sign of infection. Why might it be considered a good sign?
Further Reading / Supplemental Links
Rebecca L. Johnson and Jack Desrocher. Daring Cell Defenders. Millbrook Press, 2007.
Susan Heinrichs Gray. The Skin. Child’s World, 2005.
http://hypertextbook.com/facts/2001/IgorFridman.shtml
http://www.biocarta.com/pathfiles/h_inflamPathway.asp
http://www.nlm.nih.gov/medlineplus/ency/article/003090.htm
http://en.wikipedia.org/wiki
Vocabulary
cilia
Finger-like projects from the cells of the mucous membranes.
fever
Higher than normal body temperature.
inflammation
Reaction causing redness, warmth, and pain that occurs at the site of an infection or injury.
mucus
Sticky, moist substance that coats mucous membranes.
phagocytes
A type of white blood cells that travel to sites of inflammation and destroy pathogens and debris.
phagocytosis
The process by which phagocytes engulf and destroy pathogens or debris.
Points to Consider
How do you think pathogens can be recognized?
Why do you think the body needs specific defenses as well as general ones?
Lesson 21.4: Immune System Defenses
Lesson Objectives
Describe the immune system.
Explain how lymphocytes respond to pathogens.
Define immunity and vaccination.
Check Your Understanding
What are the first two lines of defense?
Give examples of pathogens.
Introduction
If pathogens manage to get through the body’s first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. It is called an immune response. The immune system has a special response for each type of pathogen.
The immune system is also called the lymphatic system. It is named for lymphocytes, which are the type of white blood cells involved in an immune response. The parts of the immune system are shown in Figure below. They include several lymph organs, lymph vessels, lymph, and lymph nodes (Figure below).
Figure 21.20
This diagram shows the parts of the immune system. The immune system includes several organs and a system of vessels that carry lymph. Lymph nodes are located along the lymph vessels.
Figure 21.21
Lymph Organs
The lymph organs are the red bone marrow, thymus gland, spleen, and tonsils. Each organ has a different function in the immune system. They are described in Figure below.
Organs of the Immune System
Figure 21.22
Red Bone Marrow
Red bone marrow is found inside many bones, including the femur shown here. Red bone marrow produces lymphocytes.
Thymus Gland
The thymus gland is in the chest behind the breast bone. It stores lymphocytes while they mature (Figure below).
Figure 21.23
Figure 21.24
Spleen
The spleen is in the abdomen below the lungs. Its job is to filter the blood. Any pathogens that are filtered out of the blood are destroyed by lymphocytes in the spleen (Figure above).
Tonsils
The tonsils are in the throat. They trap pathogens that enter the body through the mouth or nose. Lymphocytes in the tonsils destroy the trapped pathogens (Figure below).
Figure 21.25
Lymph and Lymph Vessels
Lymph vessels make up a circulatory system that is similar to the cardiovascular system, which you read about in the Cardiovascular System chapter. Lymph vessels are like blood vessels, except they circulate lymph instead of blood. Lymph is a yellowish fluid that leaks out of tiny blood vessels into spaces between cells in tissues. At sites of inflammation, there is usually more lymph in tissues. This lymph may contain many pathogens.
The lymph that collects in tissues gradually passes into tiny lymph vessels. It then travels from smaller to larger lymph vessels. Lymph is not pumped through lymph vessels like blood is pumped through blood vessels by the heart. Instead, muscles surrounding the lymph vessels contract and squeeze the lymph through the vessels. The lymph vessels themselves also contract to help move the lymph along. The lymph finally reaches the main lymph vessels in the chest. Here, the lymph drains into two large veins. This is how the lymph returns to the bloodstream.
Before lymph reaches the bloodstream, pathogens are removed from it at lymph nodes. Lymph nodes are small, oval structures located along the lymph vessels. They act like filters. Any pathogens filtered out of the lymph at lymph nodes are destroyed by lymphocytes in the nodes.
Lymphocytes
Lymphocytes are the key cells of an immune response. A photograph of a lymphocyte is shown in Figure below. The lymphocyte shown in the figure is greatly magnified. There are trillions of lymphocytes in the human body. They make up about one quarter of all white blood cells. Usually, fewer than half of the body’s lymphocytes are in the blood. The rest are in the lymph, lymph nodes, and lymph organs.
Figure 21.26
This image of a lymphocyte was made with an electron microscope. The lymphocyte is shown 10,000 times its actual size.
There are two main types of lymphocytes: B cells and T cells. Both types of lymphocytes are produced in the red bone marrow. They are named for the sites where they mature. The B in B cells stands for “bone.” B cells mature in red bone marrow. The T in T cells stands for “thymus.” T cells mature in the thymus gland. B and T cells must be “switched on” in order to fight a specific pathogen. Once this happens, they multiply and produce an army of cells ready to fight that particular pathogen.
How can B and T cells recognize specific pathogens? Pathogens have proteins that are foreign to the body. These proteins are called antigens. An antigen is any protein that triggers an immune response because it is unlike any protein that the body makes. Antigens are found on bacteria, viruses, and other pathogens. They are also found on other foreign cells that enter the body and on cancer cells.
Immune Responses
There are two different types of immune responses. One type involves B cells. The other type involves T cells. You can watch a video of both types of immune responses at http://www.dnatube.com/view_video2.php?viewkey=5ff68e3e25b9114205d4.
B Cell Response
B cells respond to pathogens and other foreign cells in the blood and lymph. Most B cells fight infections by producing antibodies. An antibody is a large, Y-shaped protein that binds with an antigen. Each antibody can bind with just one specific type of antigen. A diagram of an antibody binding with an antigen is shown in Figure below. They fit together like a lock and key. Antibodies travel through the blood and lymph, binding with any matching antigens they run into. Once an antigen and antibody bind together, they are destroyed by a phagocyte.
Figure 21.27
This diagram shows how an antibody binds with an antigen. The antibody was produced by a B cell. It binds with just one type of antigen. Antibodies produced by different B cells bind with other types of antigens.
T Cell Response
There are different types of T cells, including killer T cells and helper T cells. Killer T cells destroy infected, damaged, or cancerous body cells. How a killer T cell destroys an infected cell is illustrated in Figure below. When the killer T cell comes into contact with the infected cell, it releases poisons. The poisons make tiny holes in the cell membrane of the infected cell. This causes the cell to burst open. Both the infected cell and the viruses inside it are destroyed.
Figure 21.28
In this diagram, a killer T cell recognizes a body cell infected with a virus. After the killer T cell makes contact with the infected cell, it releases poisons that cause the infected cell to burst. This kills both the infected cell and the viruses inside it.
Helper T cells do not destroy infected or damaged body cells. However, they are still necessary for an immune response. They help by secreting chemicals that control other lymphocytes. The chemicals secreted by helper T cells “switch on” both B cells and killer T cells so they can recognize and fight specific pathogens.
Immunity and Vaccination
Most B and T cells die after an infection has been brought under control. However, some of them survive for many years. They may even survive for a person’s lifetime. These long-lasting B and T cells are called memory cells. They allow the immune system to “remember” the pathogen after the infection is over. If the pathogen tries to invade the body again, the memory cells are ready to start multiplying. They will quickly produce a new army of B or T cells to fight the pathogen. They are prepared to launch a faster, stronger attack than the first time the pathogen invaded the body. As a result, the immune system will be able to destroy the pathogen before it can cause an infection. Being able to resist a pathogen in this way is called immunity.
Immunity can also come about through vaccination. Vaccination is deliberate exposure to a pathogen in order to bring about immunity without causing disease. In vaccination, the pathogen is usually injected under the skin. However, only part of the pathogen is injected, or a weak or dead pathogen is used. This results in an immune response without causing illness. Diseases you have probably been vaccinated against include measles, mumps, and chicken pox.
Lesson Summary
The immune system includes lymph organs, lymph vessels, lymph, and lymph nodes.
B cells produce antibodies against pathogens in the blood and lymph.
Killer T cells destroy body cells infected with pathogens.
Immunity is the ability to resist a particular pathogen.
Vaccination is deliberate exposure to a pathogen in order to bring about immunity.
Review Questions
What are lymphocytes?
Describe ly
mph.
What is an antigen?
What organ produces B cells and T cells?
Define immunity.
Some children with frequent sore throats have an operation to remove their tonsils. Why might removing the tonsils lead to fewer sore throats?
How are an antigen and antibody like a lock and key?
Explain how killer T cells fight pathogens.
Helper T cells do not produce antibodies or destroy infected cells. Why are they necessary for immune responses?
If you have been vaccinated against measles, you are unlikely to ever have the disease, even if you are exposed to the measles virus. Why?
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