An allergic reaction happens when your immune system interacts with and responds to an offending agent of the outside world. In order to protect your body from bacteria and parasites, your immune system produces antibodies. When these antibodies recognize potentially harmful foreign elements, they produce an immune response. Allergies happen because your immune system has been conditioned to respond and attack harmless proteins, such as those found in grass pollen.
So when youre outside on a lovely spring day and your nose starts running or your eyes get itchy, its not really the grass pollen itself that causes these annoying symptomsits the way your immune system is set up to respond to this allergen as a threat.
Genetics do play a role in determining a tendency toward having allergies, but its also determined by your exposure to certain allergens. Although most allergies develop during ones teenage years, you could develop an allergy at any point in your life, whether youre 20 or 60 years old.