The effect of quenching agents on contact urticaria caused by cinnamic aldehyde.

Inhibition (quenching) of contact sensitization by cinnamic aldehyde (CA) reportedly occurs from eugenol (E) and d-limonene (d-L). Experimentally the former inhibited nonimmunologic contact urticaria (NICU) from CA in seven of eleven test subjects, which prompted a search for possible mechanisms, including chemical interaction, altered absorption, and anti-inflammatory activity, and competitive inhibition. Mixtures of CA and E and CA and d-L showed no chemical changes or intermolecular bonding. Absorption was not increased by cellophane tape stripping, and neither E nor d-L inhibited urticaria formation following stimulation of skin of five subjects with dermographism. Competitive inhibition at the receptor level may best explain the quenching phenomenon observed. Additional factors are presented that complicate the already numerous caveats in interpreting test results in NICU.