Clinical History:

A 40-year-old man was hiking through a dense rainforest. Twelve hours later, he noted a rash on the left upper back, which intensified over the next couple of days.

Image 7.1:

The skin findings are shown here. There is a rash localized to the left upper back and arm.

Questions:

1. What is the diagnosis?

This is a case of contact dermatitis. This person was previously sensitized to some substance in the plant that was contacted. The most typical plants that produce this reaction are poison oak and poison ivy.

2. What immunologic mechanism is operative here?

This is a localized form of type IV hypersensitivity reaction (cell mediated immunity). The same reaction occurs in the diagnostic skin test known as a PPD test for tuberculosis (tuberculin skin test) in which a "purified protein derivative" of mycobacteria is injected superficially, and in persons previously infected with Mycobacterium tuberculosis (or who have received a BCG vaccination), an area of induration with erythema develops in 24 to 72 hours.

3. What findings would you expect to see in the skin?

There is edema (giving the raised appearance to the lesions) and vascular dilation (giving the red appearance) along with mononuclear cell (macrophages and lymphocytes) infiltrates.