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 below.
Figure 24.1
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 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 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 below 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 24.2
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 below.
Figure 24.3
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 below. 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 24.4
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, which you will read about in Lesson 24.2: Immune Response.
Eosinophils and basophils are responsible for allergies, which are discussed in Lesson 24.3: Immune System Diseases. 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. This type of defense is the topic of the next lesson.
Lesson Summary
Mechanical, chemical, and biological barriers are the body’s first line of defense against pathogens.
The inflammatory response and phagocytosis by white blood cells are major components of the body’s second line of defense.
Review Questions
Identify two defenses in the body’s first line of defense.
Describe the process of phagocytosis.
How does the inflammatory response help fight infections?
Describe the roles of leukocytes in the body’s second line of defense.
Further Reading / Supplemental Links
Farrell, Jeanette, Invisible Enemies: Stories of Infectious Disease. Farrar, Straus and Giroux, 2005.
http://en.wikibooks.org/wiki/Human_Physiology/The_Immune_System
http://www.clevelandclinic.org/health/health-info/docs/0200/0217.asp?index=4857
http://library.thinkquest.org/C0115080/?c=wbc
http://www.sciencedaily.com/releases/2007/07/070711135623.htm
http://en.wikipedia.org
Vocabulary
biological barriers
Living organisms that compete with pathogens; help prevent harmful bacteria from becoming established in or on the body.
chemical barriers
Proteins that destroy pathogens at the body’s surface.
cytokines
Chemical signals used to communicate between cells.
fever
A higher-than-normal body temperatur
e that may help fight infection.
histamines
Chemicals that cause inflammation and allergies.
inflammatory response
A complex biological reaction to tissue damage; one of the first responses of the immune system to infection or injury; triggered by chemicals called cytokines and histamines.
lysozymes
Enzymes that kill pathogens by breaking open their cell walls; found in sweat, mucus, tears, and saliva.
mechanical barriers
Physically blocks pathogens from entering the body; the skin is the most important mechanical barrier.
mucus
A slimy substance secreted by mucus membranes; coats the membranes and traps pathogens.
nonspecific defenses
Defenses that are the same regardless of the type of pathogen; found in the first and second line of defense.
pathogens
Disease-causing agents, such as bacteria and viruses.
phagocytosis
The process of engulfing and breaking down pathogens and other unwanted substances.
white blood cells
Leukocytes; immune system cells that are specialized to fight infections; they identify and eliminate pathogens, debris, and abnormal body cells; leukocytes includes monocytes, macrophages, neutrophils, eosinophils, and basophils.
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 defends the body against specific pathogens.
Think about how the immune system could identify a particular pathogen.
Can you develop possible mechanisms for how these pathogens could be destroyed?
What roles do you think various cell types (such as lymphocytes) play in the specific defenses of the immune system?
Lesson 24.2: Immune Response
Lesson Objectives
Describe the lymphatic system and state its general functions in the immune response.
Explain the role of antigens in the immune response.
List the steps that occur in a humoral immune response.
Identify roles of different types of T cells in a cell-mediated immune response.
Define immunity and distinguish between active and passive immunity.
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 is often referred to as the immune response. This defense is specific to a particular pathogen, and it allows the immune system to “remember” the pathogen after the infection is over. If the pathogen tries to invade the body again, the immune system can launch a much faster, stronger attack. This lets the immune system destroy the pathogen before it can cause harm. The immune response mainly involves the lymphatic system.
Lymphatic System
The lymphatic system is a major component of the immune system. Because of its important role in the immune system, the terms "immune system" and "lymphatic system" are sometimes used interchangeably. However, as you read in Lesson 24.1, nonspecific defenses of the body include organs such as the skin, which is not part of the lymphatic system. In addition, the lymphatic system has another function not directly related to defense.
Functions of the Lymphatic System
The lymphatic system has three basic functions. The first function is related to digestion. The other functions are involved in the immune response.
The lymphatic system absorbs fatty acids after the digestion of lipids in the small intestine. It then transports the fatty acids to the bloodstream, where they circulate throughout the body.
The lymphatic system removes excess fluid from body tissues and returns the fluid to the blood. The fluid is filtered as it passes through the lymphatic system, and any pathogens it contains are destroyed before the fluid enters the bloodstream.
The lymphatic system produces lymphocytes. Lymphocytes are the type of white blood cells, or leukocytes, primarily involved in the immune response. They recognize and help destroy specific foreign invaders in body fluids and cells.
Parts of the Lymphatic System
The lymphatic system, which is shown in Figure below, consists of lymphatic organs, lymphatic vessels, lymph, and lymph nodes. Organs of the lymphatic system include the red bone marrow, thymus, spleen, and tonsils.
Red bone marrow is found inside many bones, including the hip, breast, and skull bones. It produces leukocytes.
The thymus is a gland located in the upper chest behind the breast bone. It stores and matures lymphocytes.
The spleen is a gland in the upper abdomen. It filters blood and destroys worn-out red blood cells. Lymphocytes in the spleen destroy any pathogens filtered out of the blood.
Tonsils are glands on either side of the pharynx in the throat. They trap pathogens, which are then destroyed by lymphocytes in the tonsils.
Figure 24.5
Human lymphatic system.
Lymphatic vessels make up a body-wide circulatory system, similar to the arteries and veins of the cardiovascular system. However, lymphatic vessels circulate lymph instead of blood. Lymph is fluid that leaks out of tiny blood vessels, called capillaries, into spaces between cells in tissues. At sites of inflammation, there is usually more lymph around cells, and it is likely to contain many pathogens.
Unlike the cardiovascular system, the lymphatic system does not have a pump to force lymph through its vessels. Lymph circulates due to peristalsis of lymphatic vessels and rhythmic contractions of the skeletal muscles that surround the vessels. Valves in the lymphatic vessels prevent lymph from flowing backwards through the system.
As lymph accumulates between cells, it diffuses into tiny lymphatic vessels. The lymph then moves through the lymphatic system, from smaller to larger vessels, until it reaches the main lymphatic ducts in the chest. Here, the lymph drains into the bloodstream.
Before lymph reaches the bloodstream, pathogens are filtered out of it at lymph nodes. Lymph nodes are small, oval structures located along the lymphatic vessels that 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 involved in the immune response. There are an estimated two trillion lymphocytes in the human body, and they make up about 25 percent of all leukocytes. Usually, fewer than half the body’s lymphocytes are found in the blood. The rest are found in the lymphatic system, where they are most likely to encounter pathogens.
The immune response depends on two types of lymphocytes: B lymphocytes, or B cells, and T lymphocytes, or T cells. Both types of lymphocytes are produced in the red bone marrow. The two types are named for the sites where they mature. B cells mature in the red bone marrow, and T cells mature in the thymus. Both B and T cells can recognize and respond to specific pathogens. B or T cells that respond to the body’s own molecules as though they were foreign, or “nonself,” receive a signal that causes them to die. Only those B and T cells that have shown they are unlikely to react to “self” molecules are released into the circulation.
Antigen Recognition
B and T cells do not actually recognize and respond to pathogens but to the antigens they carry. Antigens are protein molecules that the immune system recognizes as nonself. Any protein that can trigger an immune response because it is foreign to the body is called an antigen. Antigens include proteins on pathogens, cancer cells, and the cells of transplanted organs.
Antigen Receptors
Both B and T cells can “recognize” specific antigens because they have receptor molecules on their surface that bind to particular antigen molecules or pieces of antigen molecules. As shown in Figure below, the fit between a receptor molecule and a specific antigen is like a lock and key. Receptors on each B or T cell recognize and bind to just one type of antigen. The human body makes lymphocytes with receptor sites for a huge num
ber of possible antigens that may be encountered throughout a person’s life.
Figure 24.6
A receptor molecule on the surface of a lymphocyte binds to a particular antigen like a lock and key.
Activation of Lymphocytes
Before lymphocytes can function, they must be activated. Activation occurs the first time the cells encounter their specific antigens after leaving the red bone marrow or thymus. Until these circulating B and T cells have been activated, they are called “naïve” cells.
Humoral Immune Response
B cells are responsible for the humoral immune response. The humoral immune response takes place in blood and lymph and involves the production of antibodies. Antibodies are large, Y-shaped proteins called immunoglobulins (Ig) that recognize and bind to antigens. In humans (and other mammals) there are five types of immunoglobulins: IgA, IgD, IgE, IgG, and IgM. Antibodies are produced by activated B cells.
B Cell Activation
Naïve B cells are activated by an antigen in the sequence of events shown in Figure below. A B cell encounters its matching antigen and engulfs it. The B cell then displays fragments of the antigen on its surface. This attracts a helper T cell (which you will read about below). The helper T cell binds to the B cell at the antigen site and releases cytokines. As you read in Lesson 24.1, cytokines are chemical signals used to communicate between cells. Cytokines from the helper T cell stimulate the B cell to develop into plasma cells or memory cells.
CK-12 Biology I - Honors Page 118
