Induction of persistent tolerance to urushiol in humans

Young nonimmune subjects were injected intramuscularly multiple times with small amounts of urushiol oil derived from poison oak. One week after the last injection this group and another group of age-matched controls were exposed to a sensitizing dose of the oil painted on the skin. All were challenged at 1 mo, then each year for 7 yr. Initially all experimental subjects were tolerant to high challenge doses; this persisted for 1 yr, at which time experimental subjects were no longer completely tolerant but were clinically tolerant (hyporeactive). This state persisted for at least 4 yr, and this represents complete tolerance followed by long-lasting hyposensitization.