As many as 80% of CFS patients have a history of allergies, compared to 15% of the general population. When an allergic reaction occurs, the IgE antibody binds to the allergen (the foreign substance that induces the allergy), causing a release of histamine. Histamine is responsible for many of the symptoms related to allergy, such as fatigue, headaches, respiratory problems, and so on. Since such a high percentage of CFS patients have allergies, it may be possible that their immune systems are overreacting to the presence of general infectious agents in the same way they overreact to the presence of allergens.
(Source: Chronic Fatigue Syndrome: A complete guide to symptoms, treatments, and solving the practical problems of CFS. By Gregg Charles Fisher, with contributions by Paul Cheney, M.D., Ph.D., Nelson Gantz, M.D., David Klonoff, M.D., and James Oleske, M.D.)