PRINCIPLES OF TOXICOLOGY

162 DERMAL AND OCULAR TOXICOLOGY
can 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 antigen
allows the cascade of events to proceed much faster.
Table 8.3 lists some of the most common agents that trigger contact dermatitis. The actual number
of potential allergenic agents is almost limitless. Individual sensitivity to a particular allergen varies
greatly and is dependent on many factors, as discussed for irritant contact dermatitis. The genetic
makeup of the person probably plays the greatest role in determining whether a response occurs. This
is 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 contact
dermatitis may be sensitive. Unfortunately, the test is usually limited to agents that are the most frequent
causes of allergic contact dermatitis. As such, identifying sensitivity to an agent unique to a given
occupation may be impossible. Patch testing should be performed by physicians trained and experienced
in the technique, its pitfalls, and the subtleties of interpretation. If a compound is identified as
allergenic, the sensitive individual can attempt to avoid exposure to that agent. The distribution of the
allergic 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 indicate
an allergy to a nickel-containing pants button. A variety of treatments are used to help alleviate contact
dermatitis. The best treatment, however, is avoidance of the allergen or irritant. Baths and wet
compresses, antibiotics, antihistamines, and corticosteroids are used in various combinations to treat
contact dermatitis.
A unique situation arises when a contact allergen is ingested or enters the systemic circulation. The
most serious effects include generalized skin eruption, headache, malaise, and arthralgia. Flaring of a
previous contact dermatitis, vesicular hand eruptions, and eczema in flexor areas of the body may be
less dramatic disturbances. Systemic exposure can trigger a delayed type IV hypersensitivity reaction
with subsequent deposition of immunoglobulins and complement in the skin, which are potent inducers
of the secondary inflammatory response. Therefore, systemic exposure to a contact allergen may induce
a widespread delayed type IV hypersensitivity reaction that is not localized to one area of the body.
Ulcers
Some chemicals can cause ulceration of the skin. This involves sloughing of the epidermis and damage
to the exposed dermis. Ulcers are commonly triggered by acids, burns, and trauma and can occur on
TABLE 8.3 Commonly Encountered Contact Allergens
Source Allergen(s) Examples
Plants and trees Rhus Poison oak and ivy
Metals Nickel and chromium Earrings, coins, and watches
Glues and bonding agents Bisphenol A, formaldehyde, acrylic
monomers
Glues, building materials, and paints
Hygiene products and topical Bacitracin, neomycin,
Creams, shampoos, and topical
medications
benzalkonium chloride, lanolin,
benzocaine, and propylene glycol
medications
Antiseptics Chloramine, glutaraldehyde,
thimerosal, and mercurials
Betadine
Leather Formaldehyde and glutaraldehyde
Rubber products Hydroquinone, diphenylguanidine,
and p-phenylenediamine
Rubber gloves and boots