In this thesis the term allergy is used to denote reactions that are mediated by the antibody IgE. Such allergic reactions usually take place within half an hour after ingestion and may include itching of the mouth, swelling of the throat, hives, swelling of the face, stomach ache, vomiting, diarrhea, hay fever symptoms, obstructed breathing, and shock. In patients who develop allergic symptoms after eating certain foods, IgE specific for the offending food can be observed, which is called sensitization. Sensitization to hazelnut and peanut is common during childhood, especially in children with eczema. However, children who do not have symptoms after eating hazelnut or peanut may also be sensitized. On the other hand, young children who have never eaten hazelnut or peanut still may have become sensitized. A challenge, preferably a double-blind placebo-controlled food challenge (DBPCFC) with the relevant food is then indicated to diagnose food allergy with certainty. This thesis shows that oral challenges remain an important tool in ascertaining hazelnut and peanut allergy. It also appears that after the challenge, regardless of the results, parents feel less anxious about allergic reactions. To refine the diagnosis of hazelnut and peanut allergy, sensitization to specific allergens in hazelnut and peanut has been studied in the challenged children. It is demonstrated that in hazelnut allergy the specific allergen that is recognized is associated with the clinical severity of the allergy, while for peanut allergy it is the number of allergens recognized that seems to be paramount. In order to investigate possible therapies for peanut allergy, probiotics have been added to the diets of allergic children. However, probiotics do not change the inadequate immune response to peanut. Subsequently the activation of T cells by peanut allergens and possible T cell epitopes (an epitope is a recognition site on an allergen) has been studied in relation to IgE binding to these epitopes. This is an important issue when it comes to developing peanut-specific immunotherapy. Stimulation of T cell reactivity is necessary for immunotherapy to be successful. However, using intact peanut allergens for immunotherapy is too dangerous at present because severe IgE-mediated reactions can occur. If the B cell response (IgE reactivity) can be reduced by using a selection of non-IgE binding peptides, the side effects of such a treatment may be reduced to a considerable extent. The study showed that the epitopes that induced a T cell response were indeed only partially overlapping with IgE-binding epitopes. These promising results may allow the development of safe allergen-specific vaccines for the treatment of peanut allergy
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