What is the primary characteristic of contact dermatitis?

The primary characteristic of contact dermatitis is that it manifests as a rash due to an allergic reaction or irritation from direct contact with a substance. This condition can occur when the skin reacts negatively to allergens such as certain metals, chemicals, or plants like poison ivy. The immune system responds to these substances, leading to symptoms such as redness, itching, and swelling in the affected area.

Allergic contact dermatitis specifically involves a sensitization process where the immune system becomes reactive to a particular substance upon first exposure, causing a rash upon subsequent exposures. The key aspect here is the direct relationship between the skin and an external irritant or allergen, highlighting the condition's inflammatory nature linked to the immune response rather than infections, genetic conditions, or purely dryness of the skin.