Differences between Allergic and Irritant Contact Dermatitis

Irritant contact dermatitis

Allergic contact dermatitis

Injury is due to inherent characteristic of a compound.
Rapid Evolution usually few hours after exposure (TEMPORAL RELATION)
Prior sensitization is not required.
Dependent on concentration of agent and state of skin barrier; occurs only above threshold level
Lesions are sharp, strictly confined to the site of exposure. in general, there is an absence of vesiculation (only very strong irritants produce vesicles) and burning exceeds itching.
Decreasing the duration and frequency of contact with irritants may improve symptoms
Mostly self–diagnosis (less commonly presented to dermatologists)
Most common irritants encountered are chronic wet work, soaps, detergents, occupational exposure (cement workers)

Injury is due to antigen specific immune response. A manifestation of delayed type hypersensitivity.
Not so rapid in evolution (which may take as little as 12 h or as much as 72 h to develop.)
Prior exposure to the offending agent is necessary to develop the hypersensitivity reaction
Relatively independent of amount applied, usually very low concentrations sufficient, but depends on degree of sensitization
Lesions spread from site of exposure to the periphery; usually tiny papules; may become generalized
Requires complete avoidance of allergen to clear lesions
Diagnosed by dermatologist with the help of SKIN PATCH TEST (Gold standard)
Most common Allergens encountered are Hair product allergens, antibiotics (Neomycin, bacitracin), Preservatives (formaldehyde), metals (Nickel, cobalt, chromium)

Dermatology