'Pollen, mildew, dust—to many people, they’re essentially harmless. For those with allergies, they are the triggers that send their immune system into overdrive.
Allergies are a malfunction of the immune system that causes it to be hypersensitive to stimuli. When the immune system is triggered, immunoglobulin E (IgE), a type of antibody, signals for the release of histamines.
Histamines, which tell mucous membranes to produce mucus and cause blood vessels to dilate, are the catalyst for the inflammation that produces watery eyes, runny nose, and other symptoms we refer to as allergies.'