Can I respond to a leader ability activation with a reaction (e.g., playing Hidden Blade in response to Teemo bouncing a card)?

Ruling: Yes, you can respond to a leader ability activation with a reaction, unless it is an ADD ability (like Kai'Sa or Darius).

Nuances:

• ADD abilities (Kai'Sa, Darius) cannot be responded to
• The response must be a reaction, not an action