Type IV reactions in the skin.

Delayed type (Type IV) hypersensitivity is commonly manifested in clinical practice as allergic eczematous contact dermatitis (AECD). This reaction can be transferred by lymphoid cells but not serum and parallels other T lymphocyte immune responses. Recent evidence correlates the sensitization phase of AECD with in vitro lymphocyte transformation and the elicitation phase with lymphokine activity. Although a bewildering variety of haptens may be responsible for AECD, they have in common an ability to form co-valent bonds with proteins. Patch testing with the offending antigen will reproduce the disease. Therapy of choice remains topical or systemic corticosteroids.