Immune and histopathologic examination of flea bite-induced papular urticaria

Background: Papular urticaria caused by flea bite presents clinical symptoms of hypersensitive reaction accompanied by skin lesions. Diagnostic and therapeutic approaches to the disease often go unrewarded, partly because of our incomplete understanding of the underlying immunopathogenesis. Objective: To characterize the immune response to the flea bite in patients with papular urticaria. Methods: This study included 45 randomly selected patients and 17 controls. Cutaneous allergy tests were performed. The histopathologic and immunohistochemical characteristics of cellular infiltrate in skin lesions were established. Immunoblot analysis was used to describe the specific characteristics of flea proteins recognized by IgE and IgG in patients' serum samples. Results: Cutaneous allergy test results were negative in 87% to 98% of patients and in 88% to 100% of controls. Histopathologic and immunohistochemical studies revealed a predominance of eosinophils and CD4⁺ T lymphocytes. Immunoblotting did not show significant differences in IgG response between patients and controls. IgE recognition of flea proteins appears to decrease as the disease progresses. Conclusions: Our results suggest that the clinical manifestations of papular urticaria are mediated by a complex immune response involving more than one mechanism, with evidence for both an IgE response and a cell-mediated type IV response.