To say that the immune system is the body’s policeman a typical metaphor and is a useful metaphor.
But you need to understand what kind of policeman it is. The immune system is like the British police in colonial India: they’ll keep the peace, but no respect for human rights: if one person in a village misbehaves, they’ll beat up the whole village and engage in some administrative massacres if necessary.
So, if you think about the immune system like the policeman, remember it’s more like these guys:
than a friendly neighbourhood cop:
[Images taken from a Washington State University news item about police militarization.]