Primary and Secondary Immune Responses

Although the immune response stops once the body has overcome an infection, some memory cells remain in the body. Memory cells are lymphocytes that will not respond the first time that they meet with an antigen or an invading cell but will recognize and attack that antigen or invading cell during later infections.

Memory cells are the body's long-term protection against reinfection by a pathogen. Memory cells often remain effective throughout an individual's life. Because of memory cells, a person will get most diseases only once. When exposed to a pathogen a second time, memory cells immediately recognize it and begin to divide rapidly. They eliminate the pathogen before it can produce serious illness.

figure 47-8

figure 47-8

(a) Vaccinations take advantage of the production of memory cells and the secondary immune response.

(b) Compare the production of antibodies during the primary and secondary immune responses that are shown on the graph.

Organizing the Immune Response

Materials paper, pencil Procedure Create a diagram or a flowchart that outlines the steps involved in an immune response. Label the cells and the steps. Analysis What are helper T cells? How is a cell-mediated response different from a humoral response?

Organizing the Immune Response

Materials paper, pencil Procedure Create a diagram or a flowchart that outlines the steps involved in an immune response. Label the cells and the steps. Analysis What are helper T cells? How is a cell-mediated response different from a humoral response?

Primary and Secondary Immune Responses

The first time the body encounters an antigen, the immune response is called a primary immune response. The response of memory cells to a later infection by the same pathogen is called a secondary immune response. The secondary immune response is much faster and more powerful, producing many more antibodies, as shown in the graph above. Recall that memory cells protect only against pathogens already encountered. Colds and flu are an exception, because rhinoviruses and flu viruses mutate at a high rate. Therefore, these viruses are always presenting new antigens.

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