Allergies and Hypersensitivities - Environmental allergies

Environmental agents such as sunlight, excessive cold, and pressure are known to produce allergic reactions in certain individuals. Cold allergy, for example, can result in hives and may even lead to a drop in blood pressure, fainting, severe shock, and sometimes death. Research into the causes of cold allergy has shown that cold urticaria, or hives, results from a histamine released from body tissues as they begin to warm up after a cold stimulus. Extremely high histamine levels coincide with episodes of very low blood pressure, the cause of fainting.

Although reaction of the body tissues to the invasion of microbes, such as bacteria, viruses, and other microorganisms, generally is not thought of as an allergic situation, the manner in which the body musters its defenses against the foreign materials is essentially the same as the way the antibodies are mobilized to neutralize other allergens. Thus, there is a similarity between infectious diseases and allergies.