Excerpts from Dermatology Textbooks

Champion, p. 737: 

Poison-oak urushiol is an intermediately potent sensitizer. Half of Americans become sensitive to 25 micrograms, applied once under a Band Aid, in six to thirty-five days. Increasing the dose to 50 mcg increases the number of reactors to 80 %.

After the first reaction, response to re-exposure occurs more rapidly. Twenty-four to forty-eight hours is usual.

Re-exposure sufficient to provoke a reaction boosts sensitivity. After the re-exposure, reactions begin to traces of the allergen so small they would not have previously incited a response. 

ANACARDIACEAE, pp. 63-97 and 95 in Mitchell and Rook:

Carriage by the fingers may extend the eventual distribution. It also accounts for the initial involvement of sites remote from those of greatest contact, as in lacquer dermatitis of the face in Mah Jongg players. The thicker horny layer on the fingers provides some protection.

However, by the time blisters form the haptene has polymerized and become attached to the tissues. The blister fluid is not dangerous to touch.

The eruption evolves in crops, affecting first the sites where the most allergen has been absorbed, and then successively the less heavily contaminated sites. Malaise and fever are unusual. The dermatitis commonly reaches its full extent in 48 hours.  Healing occurs within two weeks, without scars unless secondary infection occurs.

However, if the plant is eaten there may be vomiting and diarrhea, drowsiness, convulsions, delirium, fever, and pupillary dilation. Nephritis, too, may complicate poisoning, either by ingestion or after massive cutaneous vesication. References:  Rytand, DA (1948) “Fatal anuria, the anephrotic syndrome and glomerulonephritis as sequels of the dermatitis of poison oak” Am. J. Med. 5 548. Templeton, HJ et al (1947) “Poison oak dermatitis: studies on haematologic, urinary and temperature changes” J. Invest. Derm. 8 53.

Author’s Note: 

There seem to be threshold phenomena involved, determining when and whether a rash occurs. I have read that T8 lymphocytes have an anti-inflammatory influence; they delay the start of reactions, and initiate healing.