An allergy is an overreaction of the immune system to a substance that's harmless to most people. But in someone with an allergy, the body's immune system treats the substance (called an allergen) as an invader and overreacts, causing symptoms that can range from annoying to serious or life threatening.
In an attempt to protect the body, the immune system of the allergic person produces antibodies called immunoglobulin E (IgE). Those antibodies then cause mast cells and basophils (allergy cells in the body) to release chemicals (including histamine) into the bloodstream to defend against the allergen "invader."
It's the release of these chemicals that causes allergic reactions, affecting a person's eyes, nose, throat, lungs, skin, or gastrointestinal tract as the body attempts to rid itself of the invading allergen. Future exposure to that same allergen will trigger this allergic response again. This means that every time the person eats that particular food or is exposed to that particular allergen, he or she will have an allergic reaction.
Allergies can be seasonal (happening only at certain times of the year, like when pollen counts are high) or can occur any time someone comes in contact with an allergen.

Who Gets Allergies?

The tendency to develop allergies is often hereditary, which means it can be passed down through your genes. However, just because you, your partner, or one of your children might have allergies doesn't mean that all of your kids will definitely get them, too. And someone usually doesn't inherit a particular allergy, just the likelihood of having allergies.
But a few kids have allergies even if no family member is allergic. And a child who is allergic to one substance is likely to be allergic to others.