Allergies, including hay fever, occur when the immune system overreacts to allergens such as pollen, dust, or pet hair. Common symptoms include sneezing, runny nose, itchy eyes, and congestion. Hay fever is often seasonal, triggered by pollen from trees, grasses, and weeds during specific times of the year. Genetic and environmental factors play a significant role in developing allergies. Treatments include antihistamines, decongestants, and nasal sprays to manage symptoms.