FIGURE 23-38 Time course of a viral infection. The initial antiviral response, seen when the number of infectious particles rises, includes activation of natural killer (NK) cells and production of type I interferons. These responses are part of the innate immune response. The production of antibodies and the activation of cytotoxic T cells (CTLs) follow, eventually clearing the infection. Re-exposure to the same virus leads to more rapid and more pronounced production of antibodies and to more rapid activation of cytotoxic T cells. A successful vaccine induces an immune response similar in some respects to that following initial exposure to a pathogen, but without causing significant symptoms of disease. If a vaccinated person is subsequently exposed to the same pathogen, the adaptive immune system is primed to respond quickly and strongly.