From WebMD:
The rash that results from the poison plants is a form of allergic contact dermatitis. (Dermatitis is swelling and irritation of the skin.) Skin is not automatically sensitive to urushiol. Sensitivity builds up after the skin is exposed to the substance. When initially exposed to urushiol, the skin alerts the immune system of the presence of the irritating chemical. However, it's common for no visible reaction will occur the first time a person comes in contact with a poison plant. The immune system then prepares a defensive reaction for the next time the skin encounters the substance. This sensitizes the skin so that new contact with urushiol causes an allergic reaction.
From another site:
Myth 7. Some individuals are immune to poison ivy.
True, but don't take that too seriously. In a way, most of us are "immune" as youngsters. That means, we can be exposed to the plant without any significant reaction. Those early exposures, however, do make physiological changes to the immune system that are not readily apparent. T-cells, one of several specialized agents designed to fight invading substances, viruses, and bacteria, develop slowly with each exposure. Lots of exposures speed the process as we become "sensitized."
Eventually, they reach a level where they concentrate at the site of contact with uroshiol and cause the red inflammation we blame on poison ivy. Usually, we become sensitized to poison ivy in our teen years and continue to react to it thereafter.
But....... I had it really bad when I was about 8yrs old, 90% of body covered. Now I rarely break out with it, and I have had a few heavy eposures, maybe just lucky....