If you give an Allay a Totem of Undying and they use it up, you will still not be able to damage them and they will still follow you.

Steps to reproduce: 
1. Summon an Allay

2. Give him a totem in his hand

3. Let him use it, for example by an exploding Respawn Anchor

Now he no longer takes damage from the player, although he should still be able to.