Host factors involved in the risk for allergy are heredity, sex, race and age, with heredity being by far the most important. Exposure to allergens has been identified as an influential environmental factor, whereas passive smoking and pollution may act as an adjuvant. The atopic mother may during pregnancy add to an atopy-prone environment. Whereas respiratory infections are associated with attacks of bronchial asthma, infections in early life might play a role in the protection against atopy by preferential stimulation of a Th1 response, with mutual down-regulation of the Th2 response.

Conclusion: Recognition of the risk factors for allergy is important in order to select the factors that could be modified for individuals at risk and in order to identify those factors of which the modulation could evolve in general preventive measures.