4 Treatment and Prevention

Activity 3

Defending Against Infection

Students select a disease from the Timeline of Infectious Diseases and model how the immune system would respond to the pathogenic agent of that disease using information from Reading 3 (On Guard), Table 1: The Immune Response to Infection.

Encourage students to take careful notes, particularly while watching the video and to watch the videos more than once, pausing when needed.

The immune system is extremely complex. For simplicity students may choose to model only a cellular response or a humoral response for the sake of clarity and simplicity. They should be reminded that in most cases both responses are occurring. Alternatively you may want to have students play the Immune System Game, which gives students a feel for the complexity of the immune response.

In this task you will model how the immune system responds to a viral or a bacterial infection using information from the reading On Guard, the table The Immune Response to Infection, and the video The Immune Response from Reading 3.

Cellular Immune Response

The following two videos provide more detailed information about the cellular and humoral immune responses to bacteria and to viruses.

Source: Nature

Humoral Immune Response

Source: Nature

The table below provides a detailed description of the major players in the immune response to viruses and bacteria.

Table 1. The Immune System's Response to Infection.
Immune System Component Action Interacts with
  • Circulates in the blood and lymph.
  • Engulfs viruses, bacteria, infected cells that display the antigen on the surface.
  • Inserts engulfed viral antigen (protein) on their own surface membrane.
  • Signals to other immune cells (T-cells and B-cells).
  • Stimulates helper T cells to bind to the viral antigen on the macrophage surface causing the macrophage to release a protein called interleukin-1.
  • Interleukin-1 stimulates helper T cells to divide.
  • Engulfs viruses, bacteria, or infected cells coated in the antibody.
  • viruses
  • bacteria
  • an infected host cell displaying viral antigen on its surface
  • helper T cells
  • viruses, bacteria, or infected cells with antibodies bound to antigens
Helper T Cell
  • Originates in bone marrow, matures in thymus gland (thus the “T” in T cell).
  • Circulates in blood and lymph.
  • Recognizes bacteria, and viruses by the proteins found on their surfaces.
  • Recognizes viral and bacterial antigens on macrophage’s surface.
  • Binds to viral and bacterial antigens on a macrophage, causing macrophage to release a chemical substance interleukin-1, which stimulates helper T cells to divide.
  • Releases activating factor, interleukin-2, which stimulates other T cells and B cells to divide.
  • Part of cell-mediated response.
  • macrophages
  • killer T cells
  • B cells
Killer (Cytotoxic) Cell
  • Originates in bone marrow, matures in thymus gland (thus the “T” in T cell).
  • Circulates in blood and lymph.
  • Arrives at infection when macrophages send out signal.
  • Activated by interleukin released from helper T cells.
  • Destroys virus infected cells that display viral antigens on their surface by injecting toxic chemicals into them.
  • Part of cell-mediated response.

infected host cell displaying viral antigen on its surface

B Cell
  • Originates in bone marrow (thus the “B” in B cell).
  • Circulates in blood and lymph.
  • Binds to viruses or bacteria through antibodies on B-cell surface.
  • Binds to viral fragments found on infected cell surface.
  • Secretes antibodies that recognize viral and bacterial antigens.
  • Stimulated to divide by T-cell interleukin.
  • Host cell with viral antigens displayed on its surface.
  • viruses
  • bacteria
  • Produced by B cells.
  • Recognizes specific antigen.
  • Binds to viruses, preventing viruses from infecting cells.
  • Binds to viral antigens on the surface of infected host cells and tags those cells for destruction.
  • Stimulates macrophages to engulf viruses, bacteria, and cells with antigens that have antibodies bound to them.
  • viruses
  • bacteria
  • antigens on infected cells
Memory B Cell
  • Formed by B cell activated by interactions with antigens.
  • Remains after infection.
  • Responds rapidly when encountering the same antigen again. If identical viruses or bacteria infect the body again, the antibody on the memory B cell binds to the viruses or bacteria and marks them for destruction. The pathogen is destroyed before infection is established.

viruses and bacteria that have infected the body before


  1. Select a disease on the Timeline of Infectious Diseases. It can be the same one you chose for Activity 2 or a different one.
  2. Review Reading 3, Table 1 and the 3 videos. As you review, take careful notes about the actions of the immune cells and proteins that are relevant to your pathogen.
  3. Prepare a diagram, drawing, animation, cartoon or other means of displaying how the immune system fights to ward off infection of your pathogen. Figure 3 models the two types of responses, cellular and humoral, to a chicken pox infection as an example.
  4. Be prepared to share your model with the class.

Note: It is important to realize that the immune system will respond to infections, but may not be able to fight off all infections. When the infection cannot be stopped by the immune response, treatment, if it exists, must be carried out. Otherwise, the patient will die.