It begins with exposure. Even if you've been around a trigger many times before with no trouble, your body may suddenly see it as an invader. If this happens, your immune system studies the allergen and makes antibodies against it, in case the same situation happens again.
Then, the next time you come across that allergen, your immune system takes action. The antibodies recognize it and turn on special cells called mast cells.
The mast cells burst open, releasing chemicals such as histamine that cause symptoms such as swelling. Swelling in your nasal passages might cause a runny nose. Swelling in the airways could cause asthma symptoms.
Keep in mind that the amount of exposure can make a difference. If you're allergic to strawberries, you may have been able to eat one or two without symptoms. But once you eat three or four, you suddenly break out in hives. There's a tipping point -- or threshold -- for people with allergies. You can handle some exposure, but too much launches an immune system attack.
The problem is that you can’t predict how you’ll recover. So if you have a food allergy, you should avoid your trigger foods completely.