An allergy is the immune system’s over-response or over-reaction to a normally harmless substance in the environment. This substance, called an allergen, can be inhaled, ingested, or exposure can occur through any direct contact, even through the skin. Common inhalant allergens include pollens, mold spores, and animal dander. The most common ingested allergens include foods and medicines. The oil in the poison ivy plant is the classic allergen which causes allergy symptoms upon exposure to the skin.
The classic allergic reaction is mediated through an antibody called IgE. These IgE molecules are attached to the surface of Mast Cells in the skin, the airways, and in the intestines, and they are attached to blood cells called Basophils. If and when these IgE molecules come into contact with an allergen, the subsequent attachment of the antibody to the allergen initiates a cascade of events which leads to the immediate release of histamine and other chemical mediators from the mast cells and basophils. It is the histamine and other chemicals which cause the symptoms of allergy. A mild reaction may result in sneezing, runny nose, congestion. A more severe reaction could lead to a life threatening event called anaphylaxis.
Contrary to popular belief, not everyone has allergies. Allergies are, however very common, affecting approximately 20% of the population. The tendency toward allergies does seem to be hereditary. One can certainly inherit the allergic gene, although generally not a specific allergy. When one parent is allergic, each child has a 50% chance of having allergies. That risk jumps to 75% if both parents have allergies.