An allergen is a substance which comes in contact with your body and causes a reaction such as sneezing, itchy skin, or wheezing. In severe cases it may cause breathing difficulties or an anaphylactic shock where your body shuts down.
The body’s immune system looks upon the allergen as an invader in a similar way to how it reacts to bacteria and viruses. The body produces antibodies to attack the allergen which causes the release of a substance called histamine. It is histamine which leads to the allergic symptoms.