Although the role of T cells in skin contact sensitivity (CS) immune reactions has been intensely studied, much less is known about the regulatory properties of T cells in the oral mucosa. Animal experiments have shown that hapten sensitization of the ectodermal oral mucosa leads to antigen-specific hypersensitivity reactions in the skin. Furthermore, oral mucosa or skin hapten sensitization resulted in CS inflammatory reactions in the oral mucosa on challenge. The oral mucosa CS responses were similar to those found skin with regard to cell phenotypes and cytokines. CS-like reactions were also found in the oral mucosa after exposure to an irritant detergent, sodium lauryl sulfate (SLS). The oral mucosa reacted at smaller SLS doses than did skin. Ions and molecules released fron dental restorative materials (together with saliva and food and/or beverages) expose the gastrointestinal mucosa continuously over long time periods. From animal experiments we have learned that mice given antigen by gastric feeding, subsequently antigen-sensitized on skin, and finally elicited in the oral mucosa and in ear skin, showed tolerance in skin but gave simultaneous CS inflammatory reactions in the oral mucosa. Moreover, exposure of colon mucosa to antigen produced CS reactions in oral mucosa after challenge. Are there CS reactions in the oral mucosa? Clinical and experimental studies indicate that the oral mucosa can function both as induction and expression site of CS. The GI tract may be an important modifier of the CS inflammatory reactions seen in the oral mucosa.