QUANTITATIVE IMMUNOHISTOCHEMICAL DIFFERENCES IN LANGERHANS CELLS IN DERMATITIS DUE TO INTERNAL VERSUS EXTERNAL ANTIGEN SOURCES

Cutaneous hypersensitivity reactions can develop against antigens deli vered through the epidermis (contact dermatitis) or through the blood vessels (e.g., drug eruptions). On routine histology alone, it is not always possible to determine the route of the antigen. Langerhans cell s (LC) are the main antigen-presenting cells in contact dermatitis. De rmal dendrocytes (DC) are antigen-presenting cells and may be involved in dermal reactions. We tested the hypothesis that there is a differe nce between dermatitis due to external and internal antigen sources wi th regard to the number or function of LC and DC. In 85 cases of derma titis, numbers of S100 and HLA-DR reactive cells per linear millimetre of epidermis were counted. The amount of epidermal spongiosis was eva luated qualitatively. In 35 cases, the number of DC per mm(2) (as defi ned by Factor XIIIa expression) was evaluated. The patients were then divided into two groups based on whether the final clinical evaluation considered the dermatitis to be secondary to an external (35 cases) o r internal antigen (50 cases). Dermatitis due to external antigens had significantly more LC/mm and more frequent HLA-DR expression than der matitis due to internal antigens, mean +/- SEM; 21.2+/-2.04 vs. 9.1+/- 1.02 (p<0.00001) and 16.3+/-2.49 vs. 6.0+/-0.92 (p=0.0001), respective ly. Spongiosis was more marked in external antigen cases. DC were more numerous in internal than in external antigen cases, but the differen ces were not statistically significant. In our model, determination of numbers of LC/mm is the variable with the highest power to discrimina te between internal and internal sources. Quantification of HLA-DR+ LC and degree of spongiosis provide little additional discriminatory pow er.