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NIAID > Topics > Immune System

Immune System
 What is the Immune System
 Self and Nonself
 Structure
 Immune Cells and Their Products
 Immune Response
 Immunity: Natural and Acquired
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Immune System

Mounting an Immune Response

Immune System Responses

When challenged by a virus or other microbe, the immune system has many weapons to choose, including lymphokines, T cells, antibodies, B cells, killer cells, complements, and macrophages.
When challenged by a virus or other microbe, the immune system has many weapons to choose. Credit: NIAID.

B Cell Response

Antibodies are triggered when a B cell encounters its matching antigen; the B cell takes in the antigen and digests it; then it displays antigen fragments bound to its own distinctive MHC molecules. The combination of antigen fragment and MHC molecule attracts the help of a mature, matching T cell. Lymphokines secreted by the T cell allow the B cell to multiply and mature into antibody-producing plasma cells. Released into the bloodstream, antibodies lock onto matching antigens. These antigen-antibody complexes are soon eliminated, either by the complement cascade or by the liver and the spleen.
B cells are triggered to mature into plasma cells that produce a specific kind of antibody when the B cell encounters a specific antigen. Credit: NIAID.

T Cell Response

T cells are mobilized when they encounter a cell such as a macrophage or a B cell that has digested an antigen and is displaying antigen fragments bound to its MHC molecules. Lymphokines help the T cell to mature. The T cell, alerted and activated, secretes lymphokines. Some lymphokines attract immune cells—fresh macrophages, granulocytes, and other lymphocytes—to the site of infection. Yet other lymphokines direct the recruits once they arrive on the scene. Some lymphokines spur the growth of more T cells. Some T cells become killer cells and track down body cells infected by viruses.
T cells become active through a series of steps and then activate other immune cells by secreting lymphokines. Credit: NIAID.

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The print version is available: Understanding the Immune System: How It Works (PDF). All artwork is by Jeanne Kelly and may not be repurposed.

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