An allergy is a disorder of the immune system that causes symptoms, such as sneezing, itchy and watery eyes, and a runny nose. Allergens are the stimuli that cause these allergy-related symptoms. One of the most predominant allergies among the population is hay fever, which causes allergic conjunctivitis and itchiness.
Environmental or dietary factors can cause reactions to allergens. The majority of people that have allergies react to airborne particles, such as dust or pollen.
The risk of an allergic reaction is dependent on either host or environmental factors. An allergy is dependent on its host when the person is genetically predisposed for that allergy, either through inherited disease or congenital deficiency. An environmentally-dependent allergy is triggered when the person comes in contact with an infectious disease, an airborne allergen, pollution, or if they change their diet.
A parent with an allergy is more likely to pass it to their child, and the child's allergy is also likely to be more severe. Identical twins will share the same allergy 70% of the time.
A person is more likely to suffer from an allergy if they live in a highly industrialized country, such as factory-heavy regions in China. On the national level, allergies are more common in an individual that lives in an urban area as opposed to a rural area.
Other possibilities must be considered before an allergy is diagnosed. Some maladies share the same symptoms with certain allergies.
Skin, or puncture testing, consists of pricking a patient's arm with minimal quantities of possible allergens. The patient is diagnosed with an allergy if the corresponding needle causes an inflammatory reaction. Blood testing, although not as common as skin testing, is another way to determine if a patient is allergic to a specific substance.