Technical Abstract: Allergic reactions to peanut and tree nuts is the major cause of anaphylaxis in the U.S. In this report; structural, immunologic, and bioinformatics analysis of natural, and a recombinant variant of the major peanut allergen Ara h 1 is presented. Small angle X-ray scattering studies show that natural Ara h 1 in solution forms higher molecular weight complexes. These studies also show that the full-length recombinant protein is partially unfolded and exists as a monomer. Crystal structure of the core Ara h 1 fragment (residues 170-586) shows that the central part of the allergen has a bicupin fold which agrees with the bioinformatics analysis. In its crystal state, the core region of the Ara h 1 forms trimeric assemblies, while in solution, the protein exists as higher molecular weight assemblies. This finding shows that the residues forming the core region of the protein are sufficient for formation of Ara h 1 trimers and higher order oligomers. Natural and recombinant variants of proteins tested using in vitro gastric and duodenal digestions assay show that the natural protein is the most stable form, followed by the recombinant core Ara h 1 fragment, and the full-length recombinant protein. Additionally, IgE binding studies reveal that the natural and recombinant allergens have different patterns of interactions with antibodies. Molecular basis of cross-reactivity between vicilin allergens is also explained.