Our immune system is designed to protect the body against foreign substances. But sometimes this system overreacts by detecting harmless substances as threatening.
A person without allergies would have no reaction to this substance. But an allergic person reacts to the usually harmless substance and their body produces antibodies to attack it. This sets off a complex chain reaction which causes the release of a chemical called histamine. It is this chemical which causes the typical symptoms of an allergy, such as sneezing, itchy eyes or a runny nose.
For example, in hayfever, the immune system overreacts to pollen, and the release of histamine causes symptoms like sneezing, runny nose, itchy nose and watery eyes.