What causes them?
Seasonal Allergies happen when your body’s immune system overreacts, perceives a threat that isn’t there and attacks it. This causes the symptoms that we usually see: runny nose, itchy eyes, sneezing, sore or scratchy throat. It’s all a result of your body fighting nothing.

How do I know if I have them?
People usually get seasonal allergies under certain conditions, like from plant pollen or fungi. These pollens and fungi usually only come out during certain times of the year, hence the name “Seasonal” Allergies. Seasonal allergies usually are hereditary, meaning they run in families. If someone in your family gets allergies every season, you might have it. While these allergies usually last for your entire life, they typically get better over time.

How can I make life with allergies easier?
Medicines designed to combat mild allergies (called antihistamines), can come in a pill, a spray, a eye drop, or a liquid. Taking these medicines daily can your body against allergies. However, some of these drugs can cause drowsiness, so it’s best to talk with your doctor about which one to take for your specific allergies.

source: https://youngwomenshealth.org/2018/08/01/seasonal-allergies/