A healthy person's immune system protects against threats that enter the body, either through our breathing, eating, or touching. However, a hypersensitive immune system attacks harmless bodies (i.e. allergens) which enter the body.
When this happens the body in effect attacks itself. The immune system's mast cells secrete histamine which is supposed to attack enemies. Histamine is a powerful chemical which can affect the gastrointestinal tract, the respiratory system, skin or the cardiovascular system.
Histamine attacks the body it is supposed to protect
In an allergic reaction, the immune system improperly uses histamine to damage the bodies own tissues. The allergy symptoms you have depend on where in the body the histamine is released. If histamine is released in the skin, hives or a rash result. If it is released in the ears, nose and throat, you may have an runny nose or trouble breathing or swallowing. If histamine is released in the gastrointestinal tract, you likely will develop stomach pains, cramps or diarrhea.
Many people experience a combination of symptoms as the food is passes through the digestive system. We see that histamine can have a variety of effects on the body.