Originally posted by Jonkst
That's actually a very interesting idea... Currently, antibodies exist for caffeine, and if used correctly, with certain amplifications that I don't want to get into, they can detect it down to sub-pico molar concentrations (pretty ridiculous, eh?) It was explained to me as you could take a can of coke, pour it into a swimming pool, then take a cup of that swimming pool and pour it in another swimming pool, and you could still detect it, or something like that. Anyhow, this would mean that they could detect the drugs for a lot longer, but they'd still go away after some time.
I kinda doubt the immune thing. If that were the case, then chronic drug users would have pretty strong immune responses to their drugs. If you developed antibodies to a drug you were taking, well, you'd get pretty sick. That and you wouldn't necessarily develop an immune response if you only took drugs occasionally.
I did know a lady once who was allergic to pot though, so I guess its possible. It was not a pretty site, she warned all of her students that if they smoked and then came into contact with her she'd break into hives.