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There are two basic forms of plant rashes that affect the skin: allergic or irritant. Both are forms of contact dermatitis. Allergic contact dermatitis occurs when a sensitive person comes into contact with a plant or plant part (such as lilies or poison ivy) that acts as an allergen (a substance that triggers an exaggerated immune system response). The rash develops as part of the allergic reaction to the allergen.

Because allergic contact dermatitis involves an allergic reaction, an initial exposure to these plants rarely produces symptoms. The skin first must become sensitized to the resin released from these plants before symptoms appear. In addition, sensitivity to the resin of these plants tends to decline with age.

Allergic skin reactions can also be triggered by burning plants that cause allergic rashes. For example, poison ivy plants have oil called urushiol present in all plant parts. This oil may trigger a severe allergic reaction in people that breathe in the burned plant particles. The small burned particles can be inhaled and cause allergic contact dermatitis inside the nose, throat and lungs.

Irritant contact dermatitis occurs when someone comes into contact with a plant or plant part that irritates the skin, causing a rash. Stinging nettles are a good example of a plant that will cause an irritating rash in most people. Irritant contact dermatitis does not require developing a sensitivity to the plant.