OVERVIEW OF CONTACT DERMATITIS
Contact dermatitis is an umbrella expression for a group of dermatoses that are initiated by the pivotal event of the epidermis coming into contact with a triggering chemical. For practical purposes, there are three main clinical forms: (1) irritant contact dermatitis (ICD); (2) contact urticaria (CU); and (3) allergic contact dermatitis (ACD). Approximately 80% of contact dermatitis cases are identified as ICD, because ICD represents a nonspecific inflammatory response to a chemical when the skin barrier function is impaired. Wet work (immersing in detergents, water, or other activities that require frequent hand washing) predisposes an individual to these irritant-type reactions because of disruptions in skin barrier function (see Chapter 11). Irritancy can also occur after chronic exposure to an environment with low humidity,1 or chronic exposure to saliva (lip smacking), urine, or feces. Another example of an inducible ICD is epidermal keratinocyte damage following a cosmetic peel (Fig. 18-1).
Glycolic acid is an irritant that can result in keratinolysis. In this figure, pink tender plaques occurred on the cheeks 1 day after a 30% glycolic acid peel.
At the other end of the spectrum is CU, which accounts for approximately 0.5% of the contact dermatitides. This type of reaction is IgE-mediated and represents an immediate-type hypersensitivity response. Clinically, CU manifests classic wheals and flares (hives); with extreme cases the clinical symptoms may progress to severe respiratory compromise, anaphylaxis, and death. A primary example is latex hypersensitivity.
The pathophysiology of ACD is remarkably different from the other types of contact dermatitis. Like CU, ACD is an immunologic reaction; however, unlike CU, ACD is a consequence of lymphocyte activation (a T-cell mediated Type IV delayed-type hypersensitivity [DTH] reaction). To assist with the visualization of the sensitization process, it can be useful to consider the triggering of ACD as similar to serial vaccination, although scientifically it is important to note that they are different immunologic processes. That is, with each subsequent dose of a chemical the ability to remember that chemical for future interactions becomes more likely. For example, with the hepatitis B vaccine, three shots are required for establishing long-lasting and effective “immunity,” or memory of that chemical; conversely, a tetanus vaccine must be “boosted” to guarantee memory. Like the tetanus, the more potent chemicals may only require a single dose, such as poison ivy. In most cases of ACD, however, the “shots” are mini-doses that taken sequentially over a given period of time result in the individual being sensitized to that chemical.
STEPS LEADING TO ALLERGIC CONTACT DERMATITIS
The chemicals likely to elicit an ACD are generally small lipophilic compounds to which an individual is routinely exposed. These chemicals usually have a molecular weight less than 500 Da allowing them to penetrate the skin or ...