An allergic reaction is the process your body goes through when it senses the presence of an allergen. The allergen may be ingested as food or medication, inhaled through the mouth or nose, or come in contact with the skin. When the body senses the presence of an allergen, antibodies are produced. The antibodies attach to mast cells, which release a substance called histamine. Histamine's job is to narrow the bronchial tubes in the lungs, dilate blood vessels, and produce secretions, such as mucus.
Histamine is the main substance responsible for the symptoms characteristic of an allergic reaction.
Medications that inhibit the release of histamines are called anti-histamines. An allergic reaction is a hypersensitive reaction to a substance to which most people do not react. The exception is anaphylaxis, which is a life-threatening emergency, requiring immediate medical attention. In a mild reaction, the symptoms do not spread to other parts of the body. Symptoms of a moderate reaction In a moderate reaction, symptoms become stronger, and may start to spread throughout the body.