People with seasonal allergies have them because their immune system mistakes pollen as an intruder and attacks, eventually releasing histamines into the system, resulting in runny noses, itchy eyes, etc. Allergies are one example of an out-of-balance immune response.
After seeing this up-close picture of pollen today, I can understand why those immune cells want to attack. Wow. Nasty. Yet, kinda artistically impressive, too.