How do game engine entities communicate?

Two use cases:

1. How would entity_A send a take-damage message to entity_B?
2. How would entity_A query entity_B's HP?

Here's what I've encountered so far:

• Message queue
  
  1. entity_A creates a take-damage message and posts it to entity_B's message queue.
  2. entity_A creates a query-hp message and posts it to entity_B. entity_B in return creates an response-hp message and posts it to entity_A.
• Publish/Subscribe
  
  1. entity_B subscribes to take-damage messages (possibly with some preemptive filtering so only relevant message are delivered). entity_A produces take-damage message that references entity_B.
  2. entity_A subscribes to update-hp messages (possibly filtered). Every frame entity_B broadcasts update-hp messages.
• Signal/Slots
  1. ???
  2. entity_A connects an update-hp slot to entity_B's update-hp signal.
• Something better?

Do I have a correct understanding of how these communication schemes would tie into a game engine's entity system?
How do entities in commercial game engines communicate?