system
Given the following post, respond as if you're commenting on Reddit:
human
How come when when you wipe to hard after going to the bathroom that there's blood that you never get an infection from fecal matter getting into your bloodstream?

I know very little about things of this nature but I was thinking about this the other day. I know that keeping a wound clean is a large factor in avoiding an infection but wouldn't wiping so hard you bleed get some very nasty stuff right into your bloodstream?
gpt
The immune system isn't the same everywhere in your body; it does different things in different places. Your gut-associated mucosal immune system is very well adapted to prevent this from happening. 

In a healthy intestine, the immune cells live just under the epithelial surface in an area called the lamina propria. They constantly "sample" the bacteria from the GI tract and produce IgA antibodies that match those bacterial proteins. (For reasons that would take twice as much time to explain, your body reacts differently to pathogens than to the normal flora, so this only applies to "probiotics".) IgA is a class of antibodies that don't really do a good job of promoting inflammation - but they are really good at "neutralizing" bacteria and toxins. Basically, they hang out in the mucus layer that coats the epithelium, and bind to the various surface proteins of the bacteria. It doesn't kill them very effectively, but it does prevent them from using those surface proteins to attach to the epithelial cells and invade the body. 

Okay. Now that we've got that set up.

When something goes wrong with this system, say, a small tear in the epithelium, those bacteria do gain access to the underlying tissues. However. The broken blood vessels that are the cause of the bleeding are already the site of an inflammatory/clotting response that will quickly kill and/or contain any bacteria that do make it in, and the other blood vessels aren't literally *right* under the epithelium... they are contained in the lamina propria. It's a very small distance to travel, but for a bacterial cell, it is a significant one. We've already established that your body is constantly producing antibodies and immune cells that target these bacteria, and that those immune cells live right where the bacteria have to travel through. And we know that the antibodies are non-inflammatory. What this essentially means is: when those bacteria enter the lamina propria, there's already a shitstorm waiting for them, and it's a shitstorm that won't cause inflammation. (Inflammation makes the blood vessels a little more permeable, so that immune cells can squeeze out and reach the site of infection. That can also backfire, obviously, since cells could also get *in*.)

In summary: the only bacteria that get into underlying tissues have a fully-primed and active immune response already waiting for them, and they don't make it to the bloodstream in numbers great enough to cause disease, if at all. Also, the reason you don't detect any symptoms of infection (itching, swelling, redness, pain) is that this particular immune response is not inflammatory.

Immunology is *awesome*. :)