I have always found the Third Line of Defense hard to get my head around and I know students are in the same boat.
I found this game (http://www.biomanbio.com/GamesandLabs/Physiogames/conflictimmunity.html) and used it as an introduction to Acquired Immunity. I was then able to keep referring back to the gameplay when explaining the intricacies.
I think it helped for some.
There are explanations at the start. I found that most of the class didn't pay much attention to these. Perhaps in the future I might have them take some notes on this part.