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 the chemicals in nickel, blue eye shadow, poison oak, accelerators in gloves, etc. Symptoms fully express in 24-72 hours.