The study by Lepoittevin et al1 attempts to analyze the structure-function relationships of allergic contact dermatitis to a panel of steroids. Unfortunately, their analysis is not supported by their data, and their reasoning is oversimplified. T-lymphocytes recognize peptides bound to grooves in major histocompatibility complex molecules (eg, HLA-DR or HLA-A, -B, and -C).2 Haptens are presented bound to these peptides. Thus, the entire antigen consists of a hapten bound to a peptide presented by a major histocompatibility complex molecule. The nature of an antigen is determined by multiple factors: the chemical reactivity of the hapten3; which protein the hapten binds; which amino acid residue the hapten binds4; whether the hapten is conjugated intracellularly or extracellularly; how the protein is processed5; and on which major histocompatibility complex molecule the hapten-peptide complex is presented. Characterization of hapten cross-reactivities solely on the general shape of the hapten ignores