Loeys-Dietz syndrome (LDS) is a rare vascular genetic disorder (estimated prevalence 1/25,000-1/100,000) due primarily to mutations in the Transforming growth factor beta (TGF-β) cytokine receptor 1 and 2 genes. In addition to a common vascular phenotype with Marfan syndrome (dilatation of the ascending aorta, arachnodactyly, lens dislocation), patients present specific malformations (bifid uvula, hypertelorism, tortuous arteries) and immuno-allergic manifestations (asthma, eczema, food allergy, eosinophilic esophagitis, chronic inflammatory bowel disease). Pathophysiologically, LDS appears to be associated with hyperactivation of the intracellular TGF-β signaling pathway in a manner similar to Marfan syndrome, as evidenced by increased intracellular phosphorylated Smad2/3 (pSmad2/3) in lymphocytes. The immuno-allergic complications appear paradoxical because of the major immunosuppressive role of this cytokine on lymphoid and myeloid immune lineages. The biological description of immunological abnormalities associated with LDS is based on a single 2013 study that found increased regulatory T (Treg) and Th2 lymphocyte polarizations, as well as increased circulating eosinophil and total IgE levels. In order to better understand the underlying mechanisms, the investigators propose to perform a descriptive clinical-biological study to identify and study the immune subpopulations most impacted by the causative mutations of LDS.