system
Given the following post, respond as if you're commenting on Reddit:
human
What prevents white blood cells from entering the intestines and attacking the large colonies of bacteria that exist there?
gpt
This is a great question.  The gut is compartmentalized by things called "tight junctions" that allow for water and ion transfer while keeping everything else separated.  The intestines are lined by their own mucosal immune cells that are tempered to recognize normal flora.  It isn't foolproof, and it isn't completely understood yet.  

Your immune system usually provides a series of checks/selections to eliminate overactive or self-attacking immune cells from entering circulation.  It is believed the gut has similar molecular checks and balances that develop as it interacts with gut flora (intestinal mucosa cross-talk) and this kinda sets the stage for what is considered self vs non-self.  Some research has shown that disruptions at this phase of development can lead to IM immune disfunction.

I knew a guy in grad school who was actually studying the qualitative differences of gut biome immune cells and the effects of compartmentalization.