Allergic contact dermatitis (ACD) is a T-cell mediated type IV (delayed) hypersensitivity reaction. Initial exposure sensitizes T-lymphocytes to the allergen (hapten-protein conjugate). Re-exposure triggers a memory T-cell response, releasing inflammatory cytokines (TNF-alpha, IFN-gamma, IL-17) that cause epidermal spongiosis, vesiculation, and dermal inflammation within 24-72 hours. Irritant contact dermatitis (ICD) involves direct cytotoxic damage to keratinocytes without immune sensitization. The irritant disrupts the lipid barrier of the stratum corneum, causing transepidermal water loss, inflammatory mediator release, and keratinocyte apoptosis. ICD is dose-dependent and occurs on first exposure, while ACD requires prior sensitization.