Current research suggests that stress may activate immune cells in your skin, resulting in inflammatory skin disease :

Skin provides the first level of defense to infection, serving not only as a physical barrier, but also as a site for white blood cells to attack invading bacteria and viruses. The immune cells in skin can over-react, however, resulting in inflammatory skin diseases such as atopic dermatitis and psoriasis. Stress can trigger an outbreak in patients suffering from inflammatory skin conditions. This cross talk between stress perception, which involves the brain, and the skin is mediated the through the "brain-skin connection". Yet, little is know about the means by which stress aggravates skin diseases.
Researchers at University of Medicine Berlin and McMaster University in Canada hypothesized that stress could exacerbate skin disease by increasing the number of immune cells in the skin. To test this hypothesis, they exposed mice to sound stress, and found that this resulted in higher numbers of mature white blood cells in the skin. Furthermore, blocking the function of two proteins that attract immune cells to the skin, LFA-1 and ICAM-1, prevented the stress-induced increase in white blood cells in the skin. Taken together, these data suggest that stress activates immune cells, which in turn are central in initiating and perpetuating skin diseases. Fostered by the present observation, the goal of future studies is to prevent stress-triggered outbreaks of skin diseases by recognizing individuals at risk and identifying immune cells suitable to be targeted in therapeutic interventions.

(Source: Joachim RA, Handjiski B, Blois SM, Hagen E, Paus R, Arck PC: Stress-induced neurogenic inflammation in murine skin skews dendritic cells towards maturation and migration: key role of ICAM1/LFA-1 interactions. Am J Pathol 2009 173:1379-1388 )

Nickel adding to contact dermatitis diagnosis conundrum:

NEW ORLEANS -- Increases in nickel allergy have done nothing to make the diagnosis of contact dermatitis a more exact science, according to a pediatric dermatologist.
Nickel recently received special recognition from the American Contact Dermatitis Society ("Nickel Named Contact Allergen of the Year," SKIN & ALLERGY NEWS, February 2009, p. 1).
is the most frequent positive patch test allergen worldwide, and in the United States only poison ivy/oak is responsible for more allergic contact dermatitis (ACD). Nickel allergy now stands at almost 19% of all persons who are patch tested in referral centers, though the definite relevance is far less, Dr. Garner said at a dermatology update sponsored by Tulane University. Body piercing appears to be the primary risk factor for ACD today, while nickel in earrings, snaps, and belt buckles remains problematic. In one study, investigators found that 16% of blue jeans tested positive for nickel (Dermatitis 2007; 18:208-11). Coating these products with a nail hardener or a nickel guard can prevent the release of nickel.

(Source: Skin & Allergy News, V.39 I.6 )