The immune system is the bodily system that fights against potentially pathogenic organisms or substances.
The Human immune system can't fight silicon. (ENT: "Observer Effect")
In the 2350s and 2360s, a research project at the Darwin Genetic Research Station developed Human children who had an aggressive immune system, capable of attacking disease organisms before they entered a Human body. (TNG: "Unnatural Selection")
Ferengi immune systems are rather strong. According to Quark, who wasn't infected by the aphasia virus in 2369, the Ferengi immune system should not be underestimated. (DS9: "Babel")
Species 8472 have an immune system that can fend off Borg nanoprobes, thanks mostly to the dense DNA of their cells. (VOY: "Scorpion", "Scorpion, Part II")
Referenced parts of the system