When histamine is released into the lungs, mucous is secreted and the airway linings swell, which can cause breathing difficulties. If the histamine is released in the nose, it causes the nose to run and itch, and make your eyes tear. 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.