PRINCIPLES OF TOXICOLOGY - Biology East Borneo

162 DERMAL AND OCULAR TOXICOLOGYcan act as antigen-presenting cells under certain circumstances. After the sensitization process occurs,subsequent exposure to the allergenic chemical triggers the same cascade of events as described above.However, the prior sensitization reaction resulting in a population of T cells specific for the antigenallows the cascade of events to proceed much faster.Table 8.3 lists some of the most common agents that trigger contact dermatitis. The actual numberof potential allergenic agents is almost limitless. Individual sensitivity to a particular allergen variesgreatly and is dependent on many factors, as discussed for irritant contact dermatitis. The geneticmakeup of the person probably plays the greatest role in determining whether a response occurs. Thisis similar to the variability noticed among individuals for their sensitivity to IgE-mediated allergies,such as hay fever, in which some people respond while others do not.Patch testing is used to try to determine to which agent a person with suspected allergic contactdermatitis may be sensitive. Unfortunately, the test is usually limited to agents that are the most frequentcauses of allergic contact dermatitis. As such, identifying sensitivity to an agent unique to a givenoccupation may be impossible. Patch testing should be performed by physicians trained and experiencedin the technique, its pitfalls, and the subtleties of interpretation. If a compound is identified asallergenic, the sensitive individual can attempt to avoid exposure to that agent. The distribution of theallergic response on the body can also provide clues as to what the allergenic compound is. For example,linear stripes may indicate plant-induced dermatitis while a rash on the lower abdomen may indicatean allergy to a nickel-containing pants button. A variety of treatments are used to help alleviate contactdermatitis. The best treatment, however, is avoidance of the allergen or irritant. Baths and wetcompresses, antibiotics, antihistamines, and corticosteroids are used in various combinations to treatcontact dermatitis.A unique situation arises when a contact allergen is ingested or enters the systemic circulation. Themost serious effects include generalized skin eruption, headache, malaise, and arthralgia. Flaring of aprevious contact dermatitis, vesicular hand eruptions, and eczema in flexor areas of the body may beless dramatic disturbances. Systemic exposure can trigger a delayed type IV hypersensitivity reactionwith subsequent deposition of immunoglobulins and complement in the skin, which are potent inducersof the secondary inflammatory response. Therefore, systemic exposure to a contact allergen may inducea widespread delayed type IV hypersensitivity reaction that is not localized to one area of the body.UlcersSome chemicals can cause ulceration of the skin. This involves sloughing of the epidermis and damageto the exposed dermis. Ulcers are commonly triggered by acids, burns, and trauma and can occur onTABLE 8.3 Commonly Encountered Contact AllergensSource Allergen(s) ExamplesPlants and trees Rhus Poison oak and ivyMetals Nickel and chromium Earrings, coins, and watchesGlues and bonding agents Bisphenol A, formaldehyde, acrylic Glues, building materials, and paintsmonomersHygiene products and topicalmedicationsBacitracin, neomycin,benzalkonium chloride, lanolin,Creams, shampoos, and topicalmedicationsbenzocaine, and propylene glycolAntisepticsChloramine, glutaraldehyde, Betadinethimerosal, and mercurialsLeatherFormaldehyde and glutaraldehydeRubber productsHydroquinone, diphenylguanidine,and p-phenylenediamineRubber gloves and boots