It is an over-reaction by your immune system to the oils in the plant.

The healthier your immune system, the worse your reaction can be.

I carry a small bar of soap wherever I go. If I accidently find myself in a poison ivy patch, as quickly as possible I wash my exposed skin where it may have touched the plants. This often nips the reaction in the bud, or at least lessens its severity.

If you can also wash or change your clothing, that helps too.

Remember that the oils of the plant can transfer from your hands and body and clothing to anything else it touches. Then you pick up, say, your axe and the axe handle has oil from when you handled it a couple days before, you can get another rash.