Boehm believes that the anglerfish somehow boosts its own innate immune system to make up for the loss of the adaptive system.
So it hasn't actually deleted its entire immune system, like the title suggests. Just some part of it.
I mean I hated the title on sight because I don't even know what "the immune system" is. And I've thought about this a whole lot and written a few rambly blog posts about it (probably not currently online).
If you talk about "your circulatory system," you can name specific organs, etc. Same is true if you talk about "your nervous system" or "your digestive tract."
The same is not true of "your immune system."
Your skin is part of your immune system. It helps keep out invaders.
Your saliva and stomach acid are part of your immune system. They help kill microbes in your food to help make it safe for you to eat.
The mucus lining all mucus membranes in your body, including your respiratory tract and digestive tract, is part of your immune system. It helps keep out invaders.
So while it's fascinating that this fish has apparently deleted some important pieces of its immune system, I don't think it is accurate to say "it deleted its immune system" (in its entirety) because the odds are good it still has many systems that help protect it from invading microbes, starting with some kind of skin/scales/hide and including some kind of digestive function.
It's a really interesting piece, but the immune system isn't any one thing. It's everything the body does to try to keep out invaders and white blood cells are just one part of that.