Allergies are an autoimmune reaction to a substance (allergen) which can cause a wide variety of symptoms, ranging from a mild itch to anaphylactic shock. People can come into contact with an allergen through inhalation, consumption, or skin contact.
Although environmental factors do contribute, genetics are the major cause in the development of allergies —meaning that the susceptibility to suffering from allergies is generally hereditary. Usually symptoms develop from a young age and can manifest into adulthood. Symptoms of allergies come and go depending on exposure to triggers.