The investigators previously showed that both antibody class switching (from IgM to IgG, IgA or IgE) and antibody secretion are controlled by a polymorphic "3' regulatory region" (3'RR) of the immunoglobulin heavy chain (IgH) locus. Alleles of the 3'RR have shown influences on the severity and progression of IgA nephropathy (IgAN) (with an over-representation of the B allele among patients with severe kidney IgA deposits). Allele B also constitutes a risk factor for celiac disease, herpetiform dermatitis, psoriasis and rheumatoid arthritis. Since the 3'RR now appears as a crucial regulator of Ig production, we wish to check whether its genetic polymorphism might influence not only the occurrence of immunopathologic processes involving class-switched antibody deregulated production but also the severity of such diseases or the time course of their progression. We wish to focus on two conditions involving class-switched antibodies: on one hand the severe forms of IgE hypersensitivities, and on the other hand a disease involving pathogenic IgA and for which the prognosis is currently very difficult to predict at the onset of the disease: Henoch-Schonlein purpura (HSP).