Plant and pesticide allergens are known to cause contact dermatitis but are not used routinely in patch testing. Mark and associates investigated the role of plant and pesticide allergens in contact dermatitis by performing patch tests of these allergens in 26 patients who were suspected of having photodermatitis or airborne allergic contact dermatitis.

Patch and photopatch tests were performed according to the guidelines of the North American Contact Dermatitis Group. Duplicate sets of photoallergens were applied to the back to allow evaluation of the subject's response to patch tests and photopatch tests with ultraviolet B, ultraviolet A and visible light. The responses to ultraviolet light and visible light were quantified on the second day. The patches on the nonirradiated sites were uncovered after 48 hours. The dermal responses were quantified at 48 hours and 96 hours for both the nonirradiated and irradiated sites.

Responses were graded on a scale from 1+ (erythema, infiltration, possibly papules) to 3+ (bullae and ulcers). If the reactions on the patch and photopatch test sites were equivalent, the tests were interpreted as signifying allergic contact dermatitis. If a positive reaction was greater on the photopatch site than on the patch site, both allergic contact dermatitis and photoallergic contact dermatitis were considered to be present. A negative response on patch testing but a positive response on photopatch testing was interpreted as photoallergic contact dermatitis.

Of 26 patients, 12 (46 percent) had positive reactions to patch testing with the plant and pesticide allergens. In five patients, the reactions indicated allergic contact dermatitis; in another five, they indicated photoallergic contact dermatitis; and in two, they indicated both types of contact dermatitis. The allergic contact dermatitis reactions were to dandelion and tansy, and to the pesticides folpet and captafol. The photodermatitis reactions were to the same pesticides. In addition, three of the photoallergic reactions were to sun-screens, six were to fragrances and three were to antibacterial agents.

The authors believe the number of positive reactions in this study can be attributed in part to the inclusion of plant and pesticide allergens in the allergic testing series. They conclude that plant and pesticide allergens should be incorporated into the routine evaluation of photosensitive patients. Contact allergy to dandelion is an important factor in patients with summer exacerbation of dermatitis. Besides environmental exposure to plant and pesticide allergens, the increasingly common use of herbal remedies that contain plant products offers an additional method of exposure to plant allergens. The authors report that four of the 12 study patients with positive responses would not have been given the correct diagnosis if plant and pesticide allergens had been omitted from the testing panel.