Seasonal allergies, also called seasonal allergic rhinitis or hay fever, are the body’s reaction to airborne particles such as pollen or mold. This type of allergy kicks up at the times of year when certain trees, grasses, or weeds are in bloom, or when mold releases its spores into the air. A combination of genetics and interaction with environmental allergens causes some people to develop seasonal allergies. People with hay fever seem to have a hyper-alert immune system that’s always on the lookout for substances foreign to the body.