Ki-1 (CD30) expression in differentiation of lymphomatoid papulosis from arthropod bite reactions.

Lymphomatoid papulosis (LP) is an uncommon cutaneous T-cell lymphoproliferative disorder that can mimic arthropod bite reactions clinically and histologically. Erythematous, crusted papules and nodules occur mainly on extremities of young adults and, when biopsied, display a superficial and deep perivascular infiltrate characterized by atypical lymphocytes and scattered eosinophils. Arthropod bite reactions may show an identical histologic pattern. It has been suggested that up to 10% of patients with LP will eventually develop lymphoma. We examined biopsies from 10 cases of LP and six cases of arthropod bite reactions diagnosed by clinical history and prolonged follow-up, with BerH2, an antibody directed against the Ki-1 antigen (CD30) and found it to be useful in helping distinguish between the two entities. All cases of LP examined showed scattered Ki-1-positive large, atypical lymphocytes in the dermis. In the arthropod bite reactions, there was virtually no staining with BerH2 antibody. This suggests that the abnormal activated T-cells, which comprise the cellular infiltrate in LP, are not present in arthropod bite reactions.