Insect bite reactions are frequently encountered in our signing out practice especially in lesions referred from pediatric cases. In such cases one usually see epidermal changes in the form of spongiosis with exocytosis of inflammatory cells. Also spongiotic vesicles can be seen, usually large and multilocular. Edema of the papillary dermis can occur and in some cases this may lead to subepidermal separation. The dermis typically shows a superficial and deep perivascular infiltrate rich in esinophils and interstitial esinophils are characteristic.

However, sometimes the establishment of such diagnosis is difficult due to the presence of worrisome features. The most important of such features is the presence of large atypical cells among the infiltrate in cases of persistent nodular arthropod bite reactions. One then might think of MF, lymphomatoid papulosis or Hodgkin’s lymphoma. Such cells are usually CD30 positive adding to the confusion. Lymphoid follicles mimicking a B cell pattern can also be encountered. Sometimes the clinical picture is also confusing as the lesions tend to persist for many months and unless a clear history of an insect bite or scabies is given, the clinician refers these cases with a request of R/O lymphoma. Clonality studies if available will be helpful as the infiltrate in such cases is polyclonal. In circumstances where these studies are unavailable, one should depend on other clues. The heterogeneity of the infiltrate (presence of lymphocytes, histiocytes, esinophils and plasma cells) and the interstitial pattern of esinophils where esinophils appear scattered among collagen bundles are said to be re-assuring signs.

So it is important to keep the possibility of a persistent nodular arthropod reaction always in the back of our mind when seeing large atypical lymphocytes among mixed infiltrate. Careful history taking and clinical correlation may point to the insult of the insect.