For several decades now, the incidence of allergy diseases and especially asthma diseases with their considerable effects on health has been steadily increasing among children. According to recent comparative studies on the frequency of allergies in former East Germany and West Germany, this increase is related to our Western lifestyle.
HOW DO YOU BECOME AN ALLERGY SUFFERER?
Asthma, atopic dermatitis, allergic hay fever and food allergy are clinical manifestations of atopy, a genetic disposition leading to an exaggerated production of allergy antibodies (IgE), which affects at least 20 to 30% of the population. The development of allergic manifestations in an atopic person depends largely on the environmental influences to which they are exposed. For example, early exposure to high concentrations of allergens can favour sensitisation to these allergens and the development of allergic hay fever and asthma. So you become allergic if you are genetically predisposed and exposed to an environment that favours allergies.
WHEN DOES ALLERGEN SENSITISATION TAKE PLACE?
A study conducted in Germany with a large group of patients shows that allergic sensitisation can develop very early in life. For example, the frequency of sensitisation to mites increases from 2% in one-year-old children to 22% in six-year-olds, and that to pollen from 1% in one-year-olds to 13% in six-year-olds. Early sensitisation is associated with a type of immune response (Th2 polarisation) predominant in early childhood. This polarisation favours the synthesis of allergy antibodies (IgE), in contrast to the Th1 polarisation common in older children and adults. During intrauterine life, Th2 polarisation prevails at the interface of the fetal and maternal circulation, thus avoiding the perception of the foetus as a foreign body and its rejection. The blood of the umbilical cord also shows this Th2 polarisation, but in children predisposed to allergies, the atopics, it is exaggerated. Between birth and five years of age, the Th2 polarisation gradually develops into Th1 polarisation. In atopics, however, the Th2 polarisation persists. The balance between Th1 and TH2 polarisation thus changes with age, usually evolving from Th2 in infants to Th1 in adults. The environmental factors that influence this balance can promote or inhibit early sensitisation to allergens. It follows that the influence of the environment during the first years of life is crucial for the development of allergic sensitisation and clinical manifestations of allergy, although sensitisation may occur throughout life.
WHEN DO WE DEVELOP THE DIFFERENT ALLERGIC DISEASES
The different allergic diseases do not appear randomly: the first manifestation of atopy is generally atopic dermatitis, as in infancy. Asthma often begins in infancy and rhinitis (hay fever), especially pollen-related, in adolescence. There is also a sequence for sensitisation to different groups of allergens: the infant tends to be more sensitised to food allergens and the child from the age of three to inhalant allergens with a simultaneous decrease in food sensitisation. Thus, the atopic child predisposed to allergies develops a sequence of allergic manifestations called the allergic career.
CAN THIS ALLERGIC CAREER BE AVOIDED?
Since allergic sensitisation develops mainly in the first years of life and depends strongly on the environment to which the child is exposed, effective preventive measures should make it possible to reduce the incidence of allergies in the population. The effect of early prevention on delayed introduction of food allergens has been studied for several years. A diet containing hydrolysed bovine proteins has been shown to reduce the incidence of milk allergy as well as atopic dermatitis during the first months of life. However, this measure does not seem to influence the frequency of allergies in the longer term. Further studies are ongoing, especially on early exposure to infectious agents that may influence the type of response generated by cells of the immune system and thus promote or inhibit allergic sensitisation. A better understanding of the mechanisms by which the environment influences the development of allergic reactions during the first years of life should allow us to identify the exact cause(s) of the increasing incidence of allergies and improve the effectiveness of preventive measures.