Why do some people say they arent allergic to poison ivy?

Some people aren't. Because some people are not allergic to it. Allergies depend on the response of several factors of the individual immune system. For different reasons, some people develop an allergy to something and others do not. Over time with more exposure, someone previously without a problem can develop it. Once there, it almost always remains though.

Related Questions

Why do some people say they aren't allergic to poison ivy/oak/sumac? Are some people immune or just lucky?

Poison Ivy. Some people appear to be immune, others become immune. However, you can gain or lose immunity, so to assume you can't get it if you never have before is foolish. People change as they age. I would never assume that i was immune at any time no matter what my past experience was. Read more...