Type IV Hypersensitivity (Chemical Allergy)

A cell-mediated delayed hypersensitivity reaction, characterized by dermatitis, eczema, erythema, vesiculation (blisters), keratosis, hyperplasty (thickening of skin) and cracking. The area affected usually increases with repeated exposure. Examples include Type IV allergies to: poison oak, nickel, soaps and fragrances.