One of the marvels of the human body is that it can defend itself against harmful invaders such as viruses or bacteria. But sometimes the defenses are too aggressive and harmless substances such as dust, molds or pollen are mistakenly identified as dangerous. The immune system then rallies its defenses, which include several chemicals to attack and destroy the supposed enemy. In the process, some unpleasant and, in extreme cases, life-threatening symptoms may be experienced in the allergy-prone individual.