Histamines released in the skin produce hives and rashes. The number of exposures needed to produce enough antibodies to cause an allergic reaction varies. It can occur the second time a person has contact with the allergen, or it may not occur until years later, when many repeated exposures cause an increased number of antibodies. Many different things can trigger an allergic reaction. These allergens include medication, plants, pollens, animals, insect bites, food, exercise and, even, changes in temperature.
Hay fever is one of the most common allergies.
The cause of an allergy can be hard to determine at times. You can often discover the cause by noting when the symptoms occur. Symptoms that develop the same time each year are often due to grass, weed or tree pollen. Allergies also seem to run in families - parents with hay fever often have children with allergies. Individuals having an allergic reaction can have one or all of the above symptoms, and they can range from mild to very severe.