If you are exposed to a substance you're allergic to, your immune system will send out an all-points bulletin, causing your body to produce proteins called immunoglobulin E (IgE) antibodies. Their job is to fight off any foreign invader that threatens the body. During this process, your body releases histamines, chemicals that cause allergy symptoms. That's why some drugs that relieve allergy symptoms are called "antihistamines."
Less is known about the nature of chemical sensitivities and reactions, and why some people may be more prone to sensitivities than others. In some circles of medicine, chemical sensitivities are controversial. Some theories suggest that the immune systems of people with chemical sensitivities may be impaired, and that the part of the brain involved in learning, emotions, and memory, may be triggered by a chemical odor entering the nose; or that a condition called TILT (toxic-induced loss of tolerance) may make even low levels of chemicals cause reactions in some individuals.