There probably are people who're immune to DMT. But there's one thing about DMT, i've noticed, that is quite extraordinary: For me, and i have noticed this in others two, it seems that i either get enough to get very strong effects or too little to get any effects at all. As if once you are over the threshold dose, you immediately get very strong effects. Many substances have a gradual increase in effects when you increase the dose, but with DMT i get or, very strong effects, or no effects at all, but never something in between. I never get moderate DMT effects.
The only substance that also has this, but to a far less extent, is LSD. Once you have noticeable effects, they're immediately very psychedelic.
For many psychedelics the opposite is true: mushrooms and cannabis are the best examples of this: People are so familiar with moderate mushroom effects that many people even seem to think that LSD is a heavier psychedelic then shrooms. If i tell that shrooms can give a trip that is equal to DMT, most people won't believe this, because of this.
Cannabis isn't even considered to be a psychedelic by most people, for the same reason, while it can be as powerfull as LSD.