If you have hay fever, this is what happens.

• When you breathe in pollen or mold spores, your immune system makes a mistake. It reacts as if these things are trying to make you sick.¹
• This causes special immune cells to release large amounts of an antibody called immunoglobulin E (or IgE for short).
• This antibody makes your body release large amounts of powerful chemicals. The main ones are histamine, and different cytokines and leukotrienes.
• These chemicals make the lining of your nose and the back of your throat irritated and swollen. So your nose and throat make lots of watery mucus.
• The chemicals give you the common symptoms of hay fever, such as sneezing, a runny or stuffy nose, and itchy, red eyes. If your lungs are affected, you may get a cough or wheeze.
• When your body reacts this way, doctors say you're having an allergic reaction. You can get a reaction like this quite suddenly.² Doctors call the thing that makes your immune system react when it doesn't need to an allergen. So for hay fever, pollen or mold spores are the allergens.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.