Background: Allergic reactions to cow's milk are common in small children. One of the main protein allergens found in cow's milk is ?-lactoglobulin (?-Lg). Reindeer and bovine milk both contain related ?-Lg proteins, but the allergenicity of reindeer ?-Lg has not previously been studied. The purpose of this study was to analyze the immunological cross-reactivity of IgE antibodies from children with cow's milk allergy to reindeer and bovine ?-Lg. Methods: Sera from 17 children and a serum pool of 4 patients with elevated cow's milk-specific IgE were investigated. ?-Lg from bovine and reindeer milk was isolated in native form and an enzyme-linked immunosorbent inhibition assay was developed. Bovine ?-Lg was used as a capturing antigen and the inhibiting effects of reindeer and bovine ?-Lg on the IgE binding were measured. Results: Cross-reactivity patterns of bovine milk ?-Lg specific IgE to reindeer ?-Lg varied among patients. In general, reindeer ?-Lg showed significantly lower inhibition (mean 43%) of IgE binding to the capturing antigen than did bovine ?-Lg (mean 89%). In some patients, even high concentrations of reindeer ?-Lg only partly eliminated the IgE binding to bovine ?-Lg. Conclusions: The partial cross-reactivity of human anti-bovine IgE with reindeer ?-Lg suggests that it lacks important bovine epitopes and those that are recognized are only weakly bound. © 2006 Esmon Publicidad.