Why doesn't your immune system kill you?

The Immune system has particular signaling systems in which it can regulate the amount and type of bacteria in the gut in order to control the microbial environment without bacteria taking over the body. The job of the human immune system is to destroy pathogens. Using a combination of immediate responses (the innate immune system) and long-term memory (the adaptive immune system) in humans, the cells of the immune system are perfectly primed to seek out any cells that are foreign to the body.

This leads to a slight problem, because rather a lot of the cells within your body are 'Other' cells, and their existence is vital to your health. Within your stomach, and your respiratory tract, live a number of commensal bacteria, friendly and harmless that can survive quite happily inside you and help to fight against incoming pathogenic bacteria. Stripping away the bacteria in the gut leads to all kinds of problems including digestive problems and increased risk of disease-causing bacteria invading the now bacteria-free stomach.

Several notable yoghurt making companies are making a lot of money by selling you drinks with bacteria in them. They reassure you that the bacteria aren't dangerous, which is all well and good, but they never quite explain why the ingestion of many bacteria doesn't cause your immune system to have a panic attack.

The 'gut micro biome' is the collection of bacteria that start colonizing the inside of your intestines soon after birth, both from your mother, and from the general environment. It's helpful here to remember that technically your intestinal tract isn't actually inside your body. However there still is a trade off. The cells that make up the intestinal walls still need to be able to respond to bacteria, and the commensal bacteria still need to be contained. A non-regulated population of bacteria will simply keep growing until all available space is filled.

Starting with the innate immune system which works by recognizing molecules found in all pathogens (called PAMPs) these are recognized by human cells using receptors called TLRs (Toll-like receptors - long story) and lead to a signaling cascade that result in a huge number of cytokines and other inflammatory agents being released to kill the bacteria. In the gut this wouldn't just lead to the massive slaughter of the micro biome, but also to a huge amount of damage to the surrounding human cells. Enough exposure to microbial elements such as lipopolysacharrides can downregulate this response; the lipopolysacharrides, which are in the bacteria cell wall, down-regulate one of the key components of the signaling system, a molecule called IRAK1. This prevents the cell from mounting a response to the bacteria.

References:

1) https://experiencelife.com/article/good-bacteria-welcome/

2) http://www.marksdailyapple.com/gut-flora-healthy-immune-system/#axzz45H3t5gvw

3) http://www.because-science.org/blog/2015/1/27/why-dont-we-kill-our-microbiota

Written by Michael Sheikhai (group 3)