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24.2: The Immune Response

Difficulty Level: At Grade Created by: CK-12
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Lesson 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: http://www.youtube.com/watch?v=G7rQuFZxVQQ.

The types of immune responses is discussed at http://www.youtube.com/watch?v=rp7T4IItbtM.

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 this link: http://www.youtube.com/watch?v=qTXTDqvPnRk.

Structures of the Lymphatic System

Figure below 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.

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 Figure above). The filtered pathogens are destroyed by lymphocytes.


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 and in the animation at the link that follows, the fit between an antigen and a matching receptor molecule is like a key in a lock. http://www.youtube.com/watch?v=cL9KY_ECzfo

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

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 response involves mainly B cells and takes place in blood and lymph. You can watch an animation of the humoral immune response at the link below. http://www.cancerresearch.org/resources.aspx?id=586

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. 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 http://www.youtube.com/watch?v=Z36dUduOk1Y (14:13).

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

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). The complex flags the antigen-bearing cell for destruction by phagocytosis. The video at the following link shows how this happens: http://www.youtube.com/watch?v=lrYlZJiuf18.

An antibody matches only one type of antigen.

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. You can watch an animation of this type of immune response at this link: http://www.cancerresearch.org/Resources.aspx?id=588.

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. 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.

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.

Helper T cells are discussed at http://www.youtube.com/watch?v=uwMYpTYsNZM.

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. Figure below 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. You can watch an animation of the actions of cytotoxic T cells at this link: http://www.youtube.com/watch?v=8buaiYBKl7U.

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

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. Immunization is 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. You can watch an animation showing how immunization brings about immunity at this link: http://www.biosolutions.info/2009/05/vaccination.html.

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.

Lesson 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 http://www.youtube.com/watch?v=xaz5ftvZCyI (11:07).

Review Questions


1. List three parts of the lymphatic system and their functions.

2. What are antigens, and how do lymphocytes “recognize” them?

3. How do plasma cells form, and how do they help fight pathogens?

4. Describe one way that cytotoxic T cells destroy cells infected with viruses.

5. What is immunity? What role do memory cells play in immunity?

Apply Concepts

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

Think Critically

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

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

9. 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?

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