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Biology Lesson 17: The Immune System and Disease

Does this organism look like a space alien? A scary creature from a nightmare? In fact, it’s a 1-cm long worm that lives in the human body and causes serious harm. It enters the body through a hair follicle of the skin when it’s in a much smaller stage of its life cycle.

Like this worm, many other organisms can make us sick if they manage to enter our body. Fortunately for us, our immune system is able to keep out most such invaders. When you read this chapter, you’ll learn how your immune system keeps you safe from harm—including from scary creatures like this!

Section 1: Nonspecific Defenses

Section Objectives

  • 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
  • leukocyte
  • mucous membrane
  • mucus
  • pathogen
  • phagocytosis


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 (image in .pdf file). 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?

(image in .pdf file)

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 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) (image in .pdf file). Tears wash pathogens from the eyes, and urine flushes pathogens out of the urinary tract.

A sneeze can expel many pathogens from the respiratory tract.

(image in .pdf file)

Chemical Barriers

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

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.

Inflammatory Response

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 (image in .pdf file), 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 drawing shows what happens during the inflammatory response. Why are changes in capillaries important for this response?

(image in .pdf file)

The inflammatory response is discussed at:


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 (image in .pdf file).
  • A specific defense is tailored to a particular pathogen. Leukocytes involved in this type of defense are described in the next Section.

In this image, leukocytes (white) are attacking pathogens (star-shaped).

(image in .pdf file)

A summary of the nonspecific defenses can be viewed at:

Section Summary

  • 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.

Extra Practice

1. Jera cut her finger. The next day, the skin around the cut had become red and warm. Why are these signs of infection?

2. 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?

Section 2: The Immune Response

Section Objectives

  • Describe the lymphatic system and its roles in the immune response.
  • List the steps that occur in a humoral immune response.
  • Identify the roles of T cells in a cell-mediated immune response.
  • Define immunity, and distinguish between active and passive immunity.


  • active immunity
  • antibody
  • B cell
  • cell-mediated immune response
  • humoral immune response
  • immune response
  • immunity
  • immunization
  • lymph
  • lymphatic system
  • lymph node
  • lymphocyte
  • memory cell
  • passive immunity
  • T cell


Like the immune systems of other vertebrates, the human immune system is adaptive. If pathogens manage to get through the body’s first two lines of defense, the third line of defense takes over. The third line of defense is 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 response against that pathogen will be much faster and stronger.

You can watch an overview of the immune response at this link:

The types of immune responses is discussed at

Lymphatic System

The immune response mainly involves the lymphatic system. The lymphatic system is a major part of the immune system. It produces leukocytes called lymphocytes. Lymphocytes are the key cells involved in the immune response. They recognize and help destroy particular pathogens in body fluids and cells. They also destroy certain cancer cells.

You can watch an animation of the lymphatic system at:

Structures of the Lymphatic System

Figure below (image in .pdf file) shows the structures of the lymphatic system. They include organs, lymph vessels, lymph, and lymph nodes. Organs of the lymphatic system are the bone marrow, thymus, spleen, and tonsils.

  • Bone marrow is found inside many bones. It produces lymphocytes.
  • The thymus is located in the upper chest behind the breast bone. It stores and matures lymphocytes.
  • The spleen is in the upper abdomen. It filters pathogens and worn out red blood cells from the blood, and then lymphocytes in the spleen destroy them.
  • The tonsils are located on either side of the pharynx in the throat. They trap pathogens, which are destroyed by lymphocytes in the tonsils.

The lymphatic system consists of organs, vessels, and lymph.

(image in .pdf file)

Lymphatic Vessels and Lymph

Lymphatic vessels make up a body-wide circulatory system. The fluid they circulate is lymph. Lymph is a fluid that leaks out of capillaries into spaces between cells. As the lymph accumulates between cells, it diffuses into tiny lymphatic vessels. The lymph then moves through the lymphatic system from smaller to larger vessels. It finally drains back into the bloodstream in the chest. As lymph passes through the lymphatic vessels, pathogens are filtered out at small structures called lymph nodes (see Figureabove) (image in .pdf file).

The filtered pathogens are destroyed by lymphocytes.

(image in .pdf file)


The human body has as many as two trillion lymphocytes, and lymphocytes make up about 25% of all leukocytes. The majority of lymphocytes are found in the lymphatic system, where they are most likely to encounter pathogens. The rest are found in the blood. There are two major types of lymphocytes, called B cells and T cells. These cells get their names from the organs in which they mature. B cells mature in bone marrow, and T cells mature in the thymus. Both B and T cells recognize and respond to particular pathogens.

Antigen Recognition

B and T cells actually recognize and respond to antigens on pathogens. Antigens are molecules that the immune system recognizes as foreign to the body. Antigens are also found on cancer cells and the cells of transplanted organs. They trigger the immune system to react against the cells that carry them. This is why a transplanted organ may be rejected by the recipient’s immune system. How do B and T cells recognize specific antigens? They have receptor molecules on their surface that bind only with particular antigens. As shown in Figure below (image in .pdf file), the fit between an antigen and a matching receptor molecule is like a key in a lock.

An antigen fits the matching receptor molecule like a key in a lock.

(image in .pdf file)

Humoral Immune Response

There are actually two types of immune responses: humoral and cell-mediated. The latter response is described later in this section. The humoral immune responseinvolves mainly B cells and takes place in blood and lymph. You can watch an animation of the humoral immune response at:

B Cell Activation

B cells must be activated by an antigen before they can fight pathogens. This happens in the sequence of events shown in Figure below (image in .pdf file). First, 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 is further discussed below). The helper T cell binds to the B cell at the antigen site and releases cytokines that “tell” the B cell to develop into a plasma cell.

B lymphocytes are further discussed at:

Activation of a B cell must occur before it can respond to pathogens. What role do T cells play in the activation process?

(image in .pdf file)

Plasma Cells and Antibody Production

Plasma cells are activated B cells that secrete antibodies. Antibodies are large, Y-shaped proteins that recognize and bind to antigens. Plasma cells are like antibody factories, producing many copies of a single type of antibody. The antibodies travel throughout the body in blood and lymph. Each antibody binds to just one kind of antigen. When it does, it forms an antigen-antibody complex (see Figure below) (image in .pdf file). The complex flags the antigen-bearing cell for destruction by phagocytosis.

The video at the link below shows how this happens.

An antibody matches only one type of antigen.

(image in .pdf file)

Memory Cells

Most plasma cells live for just a few days, but some of them live much longer. They may even survive for the lifetime of the individual. Long-living plasma cells are called memory cells. They retain a “memory” of a specific pathogen long after an infection is over. They help launch a rapid response against the pathogen if it invades the body again in the future.

Cell-Mediated Immune Response

The other type of immune response, the cell-mediated immune response, involves mainly T cells. It leads to the destruction of cells that are infected with viruses. Some cancer cells are also destroyed in this way. There are several different types of T cells involved in a cell-mediated immune response, including helper, cytotoxic, and regulatory T cells.

T Cell Activation

All three types of T cells must be activated by an antigen before they can fight an infection or cancer. T cell activation is illustrated in Figure below (image in .pdf file). It begins when a B cell or nonspecific leukocyte engulfs a virus and displays its antigens. When the T cell encounters the matching antigen on a leukocyte, it becomes activated. What happens next depends on which type of T cell it is.

T cell activation requires another leukocyte to engulf a virus and display its antigen.

(image in .pdf file)

Helper T Cells

Helper T cells are like the “managers” of the immune response. They secrete cytokines, which activate or control the activities of other lymphocytes. Most helper T cells die out once a pathogen has been cleared from the body, but a few remain as memory cells. These memory cells are ready to produce large numbers of antigen-specific helper T cells like themselves if they are exposed to the same antigen in the future.

Helped T cells are discussed at:

Cytotoxic T Cells

Cytotoxic T cells destroy virus-infected cells and some cancer cells. Once activated, a cytotoxic T cell divides rapidly and produces an “army” of cells identical to itself. These cells travel throughout the body “searching” for more cells to destroy. Figurebelow (image in .pdf file) shows how a cytotoxic T cell destroys a body cell infected with viruses. The T cell releases toxins that form pores in the membrane of the infected cell. This causes the cell to burst, destroying both the cell and the viruses inside it.

A cytotoxic T cell releases toxins that destroy an infected body cell and the viruses it contains.

(image in .pdf file)

After an infection has been brought under control, most cytotoxic T cells die off. However, a few remain as memory cells. If the same pathogen enters the body again, the memory cells mount a rapid immune response. They quickly produce many copies of cytotoxic T cells specific to the antigen of that pathogen.

Regulatory T Cells

Regulatory T cells are responsible for ending the cell-mediated immune response after an infection has been curbed. They also suppress T cells that mistakenly react against self antigens. What might happen if these T cells were not suppressed?


Memory B and T cells help protect the body from re-infection by pathogens that infected the body in the past. Being able to resist a pathogen in this way is called immunity. Immunity can be active or passive.

Active Immunity

Active immunity results when an immune response to a pathogen produces memory cells. As long as the memory cells survive, the pathogen will be unable to cause a serious infection in the body. Some memory cells last for a lifetime and confer permanent immunity. Active immunity can also result from immunization. Immunizationis the deliberate exposure of a person to a pathogen in order to provoke an immune response and the formation of memory cells specific to that pathogen. The pathogen is often injected. However, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is typically used. This causes an immune response without making the immunized person sick. This is how you most likely became immune to measles, mumps, and chicken pox.

Passive Immunity

Passive immunity results when antibodies are transferred to a person who has never been exposed to the pathogen. Passive immunity lasts only as long as the antibodies survive in body fluids. This is usually between a few days and a few months. Passive immunity may be acquired by a fetus through its mother’s blood. It may also be acquired by an infant though the mother’s breast milk. Older children and adults can acquire passive immunity through the injection of antibodies.

Section Summary

  • The body’s third line of defense is the immune response. This involves the lymphatic system. This system filters pathogens from lymph and produces lymphocytes.
  • Lymphocytes are the key cells in the immune response. They are leukocytes that become activated by a particular antigen. There are two major type of lymphocytes: B cells and T cells.
  • Activated B cells produce antibodies to a particular antigen. Memory B cells remain in the body after the immune response is over and provide immunity to pathogens bearing the antigen.
  • Activated T cells destroy certain cancer cells and cells infected by viruses. Memory T cells remain in the body after the immune response and provide antigen-specific immunity to the virus.
  • Immunity is the ability to resist infection by a pathogen. Active immunity results from an immune response to a pathogen and the formation of memory cells. Passive immunity results from the transfer of antibodies to a person who has not been exposed to the pathogen.

A review of B cells and T cells is available at:

Extra Practice

1. If a disease destroyed a person’s helper T cells, how might this affect the ability to launch an immune response?

2. Compare and contrast humoral and cell-mediated immune responses.

3. How is active immunity different from passive immunity? Why does active immunity last longer?

4. Explain how immunization prevents a disease such as measles, which is caused by a virus.

Points to Consider

Sometimes the immune system makes mistakes and things go wrong.

  • What if the immune system responded to a harmless allergen as though it were a deadly pathogen? What might happen?
  • What if the immune system responded to normal body cells as though they were foreign invaders? Would the immune system destroy the body cells?
  • What if pathogens attacked and destroyed cells of the immune system itself? Would the immune system still be able to defend the body?

Section 3: Immune System Diseases

Section Objectives

  • Explain why allergies occur, and identify common allergens.
  • Describe how autoimmune diseases affect the body.
  • Define immunodeficiency, and list reasons for it.
  • Explain how HIV is transmitted and how it causes AIDS.


  • acquired immunodeficiency syndrome (AIDS)
  • allergen
  • allergy
  • autoimmune disease
  • human immunodeficiency virus (HIV)
  • immunodeficiency


Your immune system usually protects you from pathogens and keeps you well. However, like any other body system, the immune system itself can develop problems. Sometimes it responds to harmless foreign substances as though they were pathogens. Sometimes it attacks the body’s own cells. Certain diseases can also attack and damage the immune system and interfere with its ability to defend the body.


An allergy is a disease in which the immune system makes an inflammatory response to a harmless antigen. Any antigen that causes an allergy is called an allergen. Allergens may be inhaled or ingested, or they may come into contact with the skin. Two common causes of allergies are shown in Figure below (image in .pdf file). Inhaling ragweed pollen may cause coughing and sneezing. Skin contact with oils in poison ivy may cause an itchy rash.

Ragweed and poison ivy are common causes of allergies. Are you allergic to these plants?

(image in .pdf file)

The symptoms of allergies can range from mild to severe. Mild allergy symptoms are often treated with antihistamines. These are drugs that reduce or eliminate the effects of the histamines that cause allergy symptoms. The most severe allergic reaction is called anaphylaxis. This is a life-threatening response caused by a massive release of histamines. It requires emergency medical treatment.

You can watch an animated video about how allergic reactions occur and how antihistamines can control them at:

Autoimmune Diseases

Autoimmune diseases occur when the immune system fails to recognize the body’s own molecules as “self,” or belonging to the person. Instead, it attacks body cells as though they were dangerous pathogens. Some relatively common autoimmune diseases are listed in Table below. These diseases cannot be cured, although they can be treated to relieve symptoms and prevent some of the long-term damage they cause.

Autoimmune Diseases

Name of Disease

Tissues Attacked by Immune System

Results of Immune System Attack

Rheumatoid arthritis

tissues inside joints

joint damage and pain

Type 1 diabetes

insulin-producing cells of the pancreas

inability to produce insulin, high blood sugar

Multiple sclerosis

myelin sheaths of central nervous system neurons

muscle weakness, pain, fatigue

Systemic lupus erythematosus

joints, heart, other organs

joint and organ damage and pain

Table 24.4 Autoimmune diseases occur when the immune system attacks body cells.

Why does the immune system attack body cells? In some cases, it’s because of exposure to pathogens that have antigens similar to the body’s own molecules. When this happens, the immune system not only attacks the pathogens. It also attacks body cells with the similar molecules.


Immunodeficiency occurs when the immune system is not working properly. As a result, it cannot fight off pathogens that a normal immune system would be able to resist. Rarely, the problem is caused by a defective gene. More often, it is acquired during a person’s lifetime. Immunodeficiency may occur for a variety of reasons:

  • The immune system naturally becomes less effective as people get older. This is why older people are generally more susceptible to disease.
  • The immune system may be damaged by other disorders, such as obesity or drug abuse.
  • Certain medications can suppress the immune system. This is an intended effect of drugs given to people with transplanted organs. In many cases, however, it is an unwanted side effect of drugs used to treat other diseases.
  • Some pathogens attack and destroy cells of the immune system. An example is the virus known as HIV. It is the most common cause of immunodeficiency in the world today.


Human immunodeficiency virus (HIV) is a virus that attacks the immune system. An example of HIV is shown in Figure below (image in .pdf file). Many people infected with HIV eventually develop acquired immune deficiency syndrome (AIDS). This may not occur until many years after the virus first enters the body.

HIV is a virus that attacks cells of the immune system.

(image in .pdf file)

HIV Transmission

HIV is transmitted, or spread, through direct contact of mucous membranes or body fluids such as blood, semen, or breast milk. As shown in Figure below (image in .pdf file), transmission of the virus can occur through sexual contact or the use of contaminated hypodermic needles. It can also be transmitted through an infected mother’s blood to her baby during late pregnancy or birth or through breast milk after birth. In the past, HIV was also transmitted through blood transfusions. Because donated blood is now screened for HIV, the virus is no longer transmitted this way.

HIV may be transmitted in all of the ways shown here. Based on how HIV is transmitted, what can people do to protect themselves from becoming infected? What choices can they make to prevent infection?

(image in .pdf file)

HIV and the Immune System

HIV infects and destroys helper T cells. As shown in Figure below (image in .pdf file), the virus injects its own DNA into a helper T cell and uses the T cell’s “machinery” to make copies of itself. In the process the T cell is destroyed, and the virus copies go on to infect other helper T cells.

This diagram shows how HIV infects and destroys T cells.

(image in .pdf file)

HIV is able to evade the immune system and keep destroying T cells. This occurs in two ways:

  • The virus frequently mutates and changes its surface antigens. This prevents antigen-specific lymphocytes from developing that could destroy cells infected with the virus.
  • The virus uses the plasma membranes of host cells to hide its own antigens. This prevents the host’s immune system from detecting the antigens and destroying infected cells.

As time passes, the number of HIV copies keeps increasing, while the number of helper T cells keeps decreasing. The graph in Figure below (image in .pdf file) shows how the number of T cells typically declines over a period of many years following the initial HIV infection. As the number of T cells decreases, so does the ability of the immune system to defend the body. As a result, an HIV-infected person develops frequent infections. Medicines can slow down the virus but not get rid of it, so there is no cure at present for HIV infections or AIDS. There also is no vaccine to immunize people against HIV infection, but scientists are working to develop one.

It typically takes several years after infection with HIV for the drop in T cells to cripple the immune system. What do you think explains the brief spike in T cells that occurs early in the HIV infection shown here?

(image in .pdf file)


AIDS is not a single disease but a set of diseases. It results from years of damage to the immune system by HIV. It occurs when helper T cells fall to a very low level and opportunistic diseases occur (see Figure above) (image in .pdf file). Opportunistic diseases are infections and tumors that are rare except in people with immunodeficiency. The diseases take advantage of the opportunity presented by people whose immune systems can’t fight back. Opportunistic diseases are usually the direct cause of death of people with AIDS.

AIDS and HIV were first identified in 1981. Scientists think that the virus originally infected monkeys but then jumped to human populations, probably sometime dur

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