In moderately colored guinea-pig skin, UVB, PUVA (
psoralen plus UVA), and
allergic contact dermatitis were shown to induce visibly well-defined
hyperpigmentation that resembled the pigmentary changes observed in Mongoloid human skin. To clarify mechanisms of
allergen-induced
hyperpigmentation, we compared the effects of
allergic contact dermatitis on pigmentation by using 6 different
allergens:
dinitrochlorobenzene (
DNCB),
1-phenylazo-2-naphthol (PAN),
benzyl salicylate (BS),
jasmine oil (JO), hydroxycitronella (HC), and ylang ylang oil (YYO). The PAN-, JO-, HC-, and YYO-induced
allergic reactions caused a definite visible
hyperpigmentation that began to appear within 14 days, reaching maximum intensity about 40 days after the induction of the
allergic reaction. These hyperpigmentations were accompanied by a significant increase in the population of
dopa-positive melanocytes on day 24 following
allergic reactions. In contrast, BS- and
DNCB-induced
allergic reactions did not give rise to visibly distinct
hyperpigmentation in spite of the intensive
allergic reactions following their challenge application. In a nonsensitized group, primary
irritant reactions were induced by the application of 100% JO, but no distinctive
hyperpigmentation was found 40 days after the last application. Quantitative analysis of the number of melanophages in the dermis showed that there was a marked increase in the number of melanophages in PAN, YYO, and HC
allergy-induced hyperpigmented areas, with PAN showing a significant increase compared with those in non-treated areas of the same animals, whereas JO was associated with no such increase in hyperpigmented area, despite the stimulated pigmentation. In the case of the lack of induced
hyperpigmentation, as seen in BS and
DNCB allergy and JO irritation, there was also no substantial increase in the number of melanophages. Our findings indicate that
allergic contact dermatitis is a unique melanogenic stimulant different from UV irradiation.