Severity of allergic reactions are highly variable from one individual to another, they can range from absent to life threatening. Allergic manifestations and specifically those of anaphylactic reactions are generally attributed to an IgE-dependent activation of mast cells and/or basophils followed by the release of histamine. Recently however evidence accumulated that other pathways might similarly contribute or even trigger anaphylaxis. Moreover, while the variance in human populations is an important subject to scientific research, medical practices and public health policies typically take a 'one for all' approach to disease management and drug development. Indeed, individual heterogeneity in the immune response can have a big impact on the likelihood to respond to therapy. Because of the complexity of immune responses in the individual and within the population, it has not been possible thus far to define the parameters (genetic or environmental) that define the immune system of allergic patients and its natural occurring variability. Thanks to the efforts that have been made in the framework of the Labex "Milieu Intérieur" study genetic, immunological and environmental factors have been identified that can be linked to the heterogeneity of immune responses in healthy individuals. By comparing these already available data from healthy individuals to a novel cohort of patients with defined severe allergic manifestations, we will be able to identify for the first time immunological and environmental parameters that are common to patients with severe allergies and identify those parameters that distinguish allergic patients from the healthy donor cohort. This analysis will thus open new perspectives on deregulated immune pathways in allergic patients allowing to orient future treatment approaches. Furthermore, comparing immune responses before and after allergen-specific immunotherapy will help understanding, which changes in immune responses are causal to a successful treatment. Importantly, this analysis will shed light on the individual differences that may predict the outcome of treatment approaches and propose novel markers of its success.