recap

41.3 recap

The adaptive immune system reacts against nonself molecules called antigens. The system generates amazing diversity in both antibodies (produced by B cells) and in T cell receptors. In the primary immune response, B cells and T cells that recognize a particular antigen proliferate by clonal selection. Immunological memory prepares the body for a much stronger secondary immune response.

learning outcomes

You should be able to:

  • Distinguish between the recognition, activation, and effector phases in adaptive immunity.

  • Make and explain inferences regarding rates of infection in the context of immunological memory.

  • Summarize how vaccines provide long-lasting immunity to pathogens.

Question 1

A fellow passenger sneezes on the train and the common cold virus enters your nose. Outline how your body responds, noting the kinds and stages of the adaptive immune response.

Humoral immune response: Free virus binds to B cells already making antibodies that react with viral antigens (recognition). These B cells are selected to divide and form a clone. The clone produces more antibodies against the virus. These antibodies bind to free virus in the bloodstream, and phagocytes ingest and hydrolyze the virus–antibody complexes.

Cellular immune response: Virus infects cells in the respiratory tract. Some of these cells break down the viral proteins and present fragments on their surfaces (antigen presentation). T cells with a T cell receptor that can bind to the viral fragment bind to the antigen-presenting cells. These T cells stimulate the formation of cytotoxic T cells that can then kill the virus-infected cells.

Question 2

In 2009 the H1N1 strain of influenza spread around the world. People who had been alive at the time of the 1918 epidemic had low rates of infection. Explain this in terms of immunological memory.

The older people had memory cells that could make antibodies to bind to the 1918 flu strain. Since the 1918 flu strain was similar to the 2009 virus, these people were able to mount a strong immune response to the 2009 virus.

Question 3

How do vaccines make use of immunological memory?

Vaccines are inactive antigens that still provoke an immune response. Vaccination promotes a proliferation of clones that produce antibodies (B cells) or T cell receptors (T cells) that bind to the injected viral antigen. Some memory cells remain. These will be needed to mount a massive immune response when the virus (with antigen) infects later.

The general features of the adaptive system are that it is pre-made to respond specifically to virtually any antigen, and it involves an initial primary response to the antigen and a more powerful secondary response when the same antigen is encountered in the future. As we noted, the adaptive system has a humoral response with B cells making antibodies and a cellular response with T cells expressing specific receptors. We now turn to these two types of adaptive response, beginning with the humoral system.