While certain individuals have genetic predisposition to allergy, only those who experience repeated exposure to offending substances (called "antigens") ultimately develop symptoms. That exposure is usually through breathing, touching, or ingesting. Certain substances, because of their physical and chemical structure, are more likely to become allergens than others. Prime examples are pollens such as ragweed, dust, mold, spores, animal dander, feathers, cereal grains, some air-borne chemical pollutants, drugs and insect venoms.

When the allergic person comes into contact with an antigen, the body's immune system produces antibodies against the antigen. This antigen-antibody reaction causes nearby cells to release a myriad of natural substances, the most commonly recognized of which is histamine.

Common allergy symptoms include runny nose, itchy nose, sneezing, wheezing, cough, watery or itchy eyes, dry itchy skin, and blocked ears. Asthma is a condition which affects breathing and the lungs, often resulting in wheezing, cough and shortness of breath. Eczema is allergic inflammation of the skin with the formation of red patches, crusts and scales that itch and generally occurs from eating certain foods.

It is estimated that at least 20% of the population is likely to develop some kind of allergy. It is most common for allergies to begin in childhood, but it is quite possible for allergic symptoms to make their first appearance at any age. You're never too old to develop an allergy. With time, it is common for people to experience change in the way their other allergy symptoms affect them, especially in childhood. For example, a baby may develop colic or eczema or have recurrent ear infection; but, as it grows older, it may develop other allergic symptoms such as hay fever, ear fluid or asthma. Adults have many varied symptoms such as chronic post-nasal drip, rashes, and stomach and intestinal problems. Older patients still have a tendency to have allergic symptoms, although they may become less noticeable with maturity.