Every day we seem to be learning more and more about the relationship between the gut microbiome and immune health. For example, just last month, researchers discovered that the immune system doesn’t activate at nine weeks into gestation, as previously thought. We now know that the intestinal epithelial cells work to eradicate damaged cells long before our organs develop. (See “Innate Immunity and the Microbiota” below.)
The microbiota and the immune system work closely together to protect the body from disease, while also producing vitamins and creating new healthy tissues. This delicate interplay must be kept in balance with the selection of healthy lifestyle choices.
In this article, we’ll explore the connections between gut microbes and the immune system, how to help them remain in balance, and what it really means to get rid of bad bacteria.
Gut Microbiota and Immune System Homeostasis
Gut microbes function symbiotically with the immune system, providing balance and health. Alterations in the population of microbes in the gut can lead to immune dysregulation. We now know the critical role of gut microbes and how they have evolved over millennia. The immune system is designed to quickly eliminate pathogens and toxins while creating new healthy tissue. Patients with autoimmune conditions suffer as their immune system falsely identifies healthy tissue as a foreign body.
Thanks to advanced sequencing techniques, we can now analyze how the gut microbiota and immune system interact. Using either germ-free mice or manipulating the gut microbiome has allowed researchers to discover that the microbiota shapes the innate and adaptive immune response. The gut microbiota doesn’t just shape the local intestinal immune system, it also impacts systemic immune responses to achieve homeostasis.
Innate Immunity and the Microbiota
There are several interconnected systems that work together to provide immunity by removing pathogens and toxins from the body. Importantly, a tolerance to our own bodily tissues and beneficial bacteria must be kept intact to ward off autoimmune conditions. The cells involved in the innate immune system have co-evolved with the microbiota, as detailed below. The essential cells that protect us from diseases are all regulated by the microbiota.
There are five groups of cells involved in the innate immune system:
- Antigen-presenting cells (APCs) - A group of immune cells that present antigens to T cells and lymphocytes. The gut microbiota regulates the development of APCs.
- Neutrophils - The microbiota is recruited to regulate neutrophils, which make up 40-60% of white blood cells.
- Natural killer (NK) cells - Secrete cytokines and act on other cells such as macrophages to mount an immune response. There is much crosstalk between NK cells and the microbiota as they work together to promote health and ward off disease.
- Mast cells (MC) - A.K.A. a mastocyte or labrocyte, are cells that contain histamine and heparin, part of the immune and neuroimmune systems. MCs act at the intersections between intestinal mucosa, microbiota and the nervous system.
- Intestinal epithelium - Is a single cell layer that plays an important role in the immune system from conception, forming a physical and chemical barrier to protect intestinal mucosa, while working symbiotically with the microbiota to quickly remove damaged cells.
Note that Gamma delta (γδ) T cells are considered to be the bridge between the innate and adaptive immune system. The microbiota has been found to play a key role in maintaining the function of γδ T cells.
Adaptive Immunity and the Microbiota
The gut microbiota plays an important role in the development of white blood cells, such as T cells. The microbes Clostridia, a class of Firmicutes, are capable of promoting the induction of human regulatory T cells (Tregs). The two key types of cells involved in the adaptive immune system are:
- T cells (4 major groups) - T cells play a key role in the adaptive immune system. Th17 offers protection against parasites. Th2 regulates the immune response, Th1 protects against intracellular microbes and Treg regulates the immune response.
- B cells - Most are immunoglobulin A (IgA) secreting plasma cells, a type of antibody that plays a critical role in the immune function of mucous membranes. The microbiota is a major driver of Iga production.
Getting Rid of Bad Gut Bacteria (Hint: It Isn’t What You’d Think)
There isn’t such a thing as “bad bacteria” per se. The term “bad bacteria” refers to bacteria that do not contribute to overall health and wellbeing. What’s important is that the population of bacteria in the gut morphs, depending on the internal terrain. For example, at normal levels candida is fine. Candida is a type of fungus that bursts into action when lifestyle factors are out of balance. For people who abuse alcohol or eat too many sugary foods, however, candida overgrowth can be a serious problem. The reason that candida proliferates in this sort of environment is not that it’s necessarily “bad” or harmful. Instead, the overgrowth is due to an abundance of food that candida likes to eat. In a sense, candida is trying to help.
An individual’s diet is intricately linked to the populations of microbes in their gut. Eating an ultra-processed diet, packed with foods like snacks and sugary juice, affects the microbiota population. Energy intake from ultra-processed foods in the US is whopping 57.9%, meaning that just 42.1% comes from healthy foods. In the first comprehensive review of ultra-processed foods on the gut microbiota, researchers found that eating a diet high in ultra-processed foods changes the composition of the gut microbiota, leading to inflammation and even epigenetic changes. These changes impact the pathophysiology of disease and weight gain. Plus, the effects can be transferred to later generations.
The Bottom Line
As we learn more about the complex interplay between the immune system and gut microbes, we can take more decisive action to boost health and wellbeing. Our job is becoming increasingly directed toward boosting microbial health to ensure that all of the systems in the body function effectively. The diet plays a pivotal role in this process, by feeding health-promoting bacteria and helping to reduce bacteria that will proliferate under compromised conditions.
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