Anna Ruvinov - Department of Immunology
One of the main roles of our body’s immune system is to protect human cells from foreign invaders. Bacteria are some of the most well-known of these invaders and can frequently cause severe disease. Despite this, it is becoming very apparent that these microorganisms are vital to several body processes, and immunity is no exception.
Most bacteria in the human body are located in the intestines to assist us with digestion. Other highly-populated areas include the upper respiratory system and skin, but nearly every other system benefits from the presence of our single-celled friends. Starting in the large intestine, it is observed that a diverse colony of bacteria contributes to a variable population of CD4+ helper T cells. These T cells are critical for an effective immune response as they are necessary for the activation of other cells such as B cells and macrophages. Reactivity to the intestinal microbiome by T cells broadens the spectrum of antigens they can identify which strengthens the immune system as a whole. This offers greater protection against a large population of pathogens.
Moving out of the intestines and into the nervous system, it is clear that bacteria have a role in our brains as well. Microglia are an important neuron-supporting cell that doubles as a member of the immune system. They mainly function in engulfing pathogens and secreting protective proteins to maintain neuronal health. In studies involving germ-free mice that lack a microbiome, it was seen that their microglia were atrophied and poorly functioning. This may be due to a lack of short-chain fatty acids, an important signaling molecule produced via fermentation by intestinal bacteria.
Though it may seem counterintuitive and despite being portrayed as an enemy of humans, bacteria are crucial for the everyday function of our bodies.
References:
Zheng, D., Liwinski, T. & Elinav, E. (2020). Interaction between microbiota and immunity in health and disease. Cell Res, 30, 492–506. https://doi.org/10.1038/s41422-020-0332-7
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