In order to understand how we can fight allergy by inducing tolerance, one should first realize the mechanisms by which tolerance is built and how allergy results from an inadequate or failed tolerance induction. Failed tolerance is a core mechanism in the development of clinical allergy, which is the expression of imbalanced immunity, leading to or being associated with one or more instances of broken tolerance. Tolerance is a basic immunological mechanism, by which the immune system chooses not to respond to a particular antigen, either because this is a self-antigen or because it is harmless for the host. The development of tolerance starts before birth, with recognition and deletion of self-reacting immune cells and continues in the infant through recognition of antigens which do not possess danger signals. While, tolerance is an active process, i.e. the presence of the antigen is necessary, it is directed to a large extent by external, environmental signals present at the time of the antigen encounter.
Tolerance is the natural fate of proteins in non-allergic individuals. Even when tolerance has been broken and therefore an allergy has manifested clinically, it is possible for tolerance to be restored, either naturally, as often happens in allergens such as milk or eggs, or through guided exposure – in the context of what is called ‘Allergen Immunotherapy’ (AIT). There are several different types of immunotherapies depending e.g. on the route of antigen provision (subcutaneous, sublingual, oral, epicutaneous etc), but also on a large number of protocols, allergen modifications and adjuvants. While the main success of AIT lies with respiratory allergies (rhinitis or asthma), in the last few years there have been many attempts to use it for the treatment of food allergy, mostly through the oral route (oral immunotherapy). Most of these protocols remain semi-experimental – used mainly in a small number of patients in specialized centers. Nevertheless, we have recently seen the first standardized product to enter the market, becoming available in some countries. Tolerance induction is actively researched, has already changed the life of many patients and is even more promising for the future.