At the height of its activity, the immune response may be so pronounced that it actually does harm to the body. In fact, the typical, non-specific symptoms we experience with an acute dose of flu, such as fever, headache, enlarged tender glands, and general fatigue, are often not caused by the invading microbe itself but by the cytokines released by immune cells to fight it. On rare occasions, these immune-induced reactions may cause serious injury to internal organs, a result known as immunopathology.
