Callie: But isn’t he already immune to the powers of others?
Evan: Yeah, but he allowed himself to susceptible to her powers.
Immunity is the ability to be immune to all or specific type of powers (including their own), spells, or potions. The possessor can allow themselves to be suspectible to a person's power if they desire.
Some people only have this as a collective like the Primas Coven. They can still be weakened and hurt by great power, however. When most of their powers were absorbed, their immunity was weakened and they were harmed only a little by lesser beings.
This power can be stripped, however and they will not be immune anymore. The Bonum Coven doesn't have immunity, but they have a great level of high resistance when together shown when Matthew threw fire at Kristen and she was only knocked back a little, but not burnt. If Claire was alive, it's most likely they would have immunity.