Why did I become allergic?

1. Your genetic background plays a major role - we know that a family history of allergies or "Atopy" is highly significant, smaller families with fewer children favour the development of allergy. Males are more likely to develop allergies than females, and prenatal maternal diet and smoking seem to play a role. A number of genes linked to allergy and which carry the "allergy predisposition" have been identified on Chromosomes 5 and 11 - this is the "atopy phenotype".
2. The home environment in the first year of life is pivotal. Parental cigarette smoking triggers allergy, Infant diet and early introduction of allergenic foods play a role. Air Pollution has been implicated; early use of day-care institutions, early use of broad spectrum antibiotics and birth just before the spring pollen season all seem to promote allergic sensitisation. Factors that seem to prevent allergies developing include certain viral illnesses such as Hepatitis A and Measles exposure, living on a farm especially livestock farming, intestinal microflora such as Lactobacilli and the use of certain vaccines such as BCG. This highlights the "hygiene theory", whereby people living the so-called clean western lifestyle are at greater risk for developing allergy. Recent studies suggest that heavy exposure to dog and cat allergens in the home may actually prevent allergies developing in infants (they suggest having two or more pets in the home!)
3. And finally, modest exposure to the common aeroallergens and allergenic foods in conjunction with these other factors leads to sensitisation in early life and clinical allergy then develops. Modest early exposure seems to be the key to triggering sensitisation, as evidence now exists for very high allergen exposure during early life having a "protective" effect (for example to cats and dogs). However, minimal exposure during the first year of life is still the recommended "rule of thumb" for allergy prevention.