Both poison ivy and poison oak can cause a red rash on the skin due to an allergic reaction to the chemical urushiol in their oily sap. I've heard people say that they are allergic to either one or the other before but not both. It seems to me that since the chemical responsible for the reaction is the same in both plants one should be susceptible to both or neither at all.
Is it possible to only be allergic to one, and if so, how is this possible?