What is atopy?

Atopy is a genetically-determined predisposition to develop localised allergic reactions to inhaled or ingested allergens. Let's break that down.

1. A genetically-determined predisposition - About 50% of atopic individuals have a family member that also suffers from atopy. Without a family history, the childhood incidence of atopy is about 12.5%, but this climbs to 20% if one parent is atopic, 45% if both parents are atopic, and 30% if one sibling is atopic. If both parents are atopic and they suffer from the same specific manifestation (e.g. asthma, or allergic eczema) then the unfortunate child has over a 70% chance of developing atopy.


2. Localised - Common allergic conditions include certain forms of asthma, eczema, rhinitis (hay fever) and conjunctivitis. However, allergies can cause many other diseases, and not all causes of these conditions are due to allergies. For instance, conjunctivitis may be caused by viruses or bacteria too.


3. Allergic reactions - Note that the conditions are necessarily allergic; that is, they are primarily mediated by IgE antibodies directed at the particular allergen. Not all allergies occur in atopic people, though. It is possible to have an allergic drug reaction, for example, without being an atopic individual.