Immunity and the Gut

Seasonal Illness Surge: Cold, RSV, and norovirus are common this time of year, often linked to weakened immune systems.

The Overlooked Defender: Beyond home remedies, the gut plays a critical role in preventing infections and supporting overall immunity.

Gut-Immune Connection: Gut bacteria can help fight viruses (UMass Chan Medical School, 2021) in the gut (e.g., norovirus) and lungs (e.g., flu), showcasing the microbiome’s powerful role in immune responses.

When a potential pathogen enters the body, there are three lines of defense it has to cross in order to cause infection in the gastrointestinal tract: the intestinal microbiota, the intestinal epithelial layer and the mucosal immune system.

The intestinal microbiota are the trillions of diverse bacteria and other organisms that inhabit your gut and that are essential for your health. These organisms can protect your gut by producing natural antibiotics called bacteriocins, which kill or inhibit other bacteria. The microbiome also ferments the fiber we eat into short-chain fatty acids (SCFAs) that can produce antimicrobial peptides(Pickard et al., 2019), which fight infections. When these protective processes are disrupted by dysbiosis (a poor microbial ecosystem), harmful bacteria and viruses can overgrow, leading to inflammation, infections and other health issues.  In addition to producing these bacteriocins, the microbiota interact with our own immune system. With 70-80% of our immune cells being present in the gut, there is an intricate interplay between the intestinal microbiota and the immune system. Research has shown that bacteria in the gut activate memory T cells(Kamada et al., 2013), a subset of immune cells that "remember" past infections, providing faster, more effective responses to previously encountered pathogens and ensuring long-lasting protection, important for fighting off infections (Doherty Institute, n.d.).

The gut epithelium contains goblet cells that secrete mucin, forming a protective barrier for the epithelial lining. The inner mucus layer is free of microbes, while the outer layer contains both mucus and microbes. Bifidobacteria stimulate mucus production, while Akkermansia both degrade and promote mucus production. Many bacteria, including several pathogens, degrade the mucus layer, which can lead to suppressed production and inflammation. Disorders like inflammatory bowel disease are linked to dysfunctions in the gut’s mucus layer, and people with ulcerative colitis produce defective mucin, which contributes to the issue (ISAPP, 2023).

Gut-associated lymphoid tissue (GALT)  is a major component of mucosal-associated lymphoid tissue (MALT), accounting for nearly 70% of the entire immune system.  The GALT functions to maintain the health of your gut and overall body by supporting the development of immune cells, like B cells, which fight infections (Bemark, Pitcher, Dionisi, & Spencer, 2024). Around 80% of plasma cells, which produce antibodies (predominantly immunoglobulin A (IgA)-producing cells (Manichanh et al., 2009), are located within GALT.

Respiratory infections, like those caused by influenza and Respiratory Syncytial Virus (RSV), can significantly alter the gut microbiome. One of the SCFAs produced by the gut microbiome, Acetate, can help protect the body fight infection. Acetate boosts immune responses, especially by enhancing an infection fighting compound, type 1 interferon (Zhao et al., 2019), which is essential for fighting RSV infection and supporting overall immune health. While the specific microbial changes caused by RSV infection are still being studied, the interplay between immune signals, cytokines like interferon, and gut-derived metabolites (Hernández et al., 2023) is crucial for understanding the infection's impact on health.

If you ever have had Norovirus, you know that the symptoms can be severe. Not only is Norovirus highly debilitating, but it also stands as the leading cause of acute gastroenteritis globally. It is also known to cause chronic infections in individuals with compromised immune systems. Research revealed that when dysbiosis occurs, certain bacteria such as Faecalibacterium and Ruminococcus, can be associated with lower IgA antibody levels, potentially increasing susceptibility to Norovirus  (Cheng et al., 2017).

Antibiotics significantly impact the microbial ecosystem. To support gut health while taking antibiotics, consuming a diet rich in microbiota-accessible carbohydrates, such as legumes (beans, peas and lentils) and oats, can help protect against antibiotic-induced gut dysbiosis (AID). Research shows that timing matters, with oat supplementation during antibiotic treatment providing the greatest protection by enhancing gut resilience and minimizing microbial disruptions (Chahal et al., 2023). Similarly, dietary fiber supplements can counteract AID by modifying the gut's chemical environment, including lowering redox potential, which reflects the oxidative balance in the gut. By reducing oxidative stress—a state where for example harmful reactive oxygen species molecules outnumber protective antioxidants—fiber helps create a stable environment that supports beneficial microbes.

Preventing and controlling Norovirus outbreaks requires diverse strategies due to its various transmission modes and highly contagious nature. Hand hygiene with soap and water for at least 20 seconds is the most effective method, while alcohol-based sanitizers should only supplement hand washing. The use of disposable personal protective equipment, such as gloves and masks, is essential, particularly when managing contaminated areas or direct exposure to infected individuals (Barker, Vipond, & Bloomfield, 2015). For RSV, currently, there is no definitive treatment for RSV, and supportive therapy remains the primary approach. Supportive care, including hydration, oxygen, and ventilation, is typically provided in severe cases, while most RSV cases are managed outpatient. To prevent the spread of RSV, handwashing and social distancing are crucial during virus season (Bemark et al., 2024; PMC, 2023).

As cold and flu season brings common illnesses like RSV, Norovirus and the flu, it's important to remember the role your gut plays in staying healthy. Supporting your gut bacteria with a diet rich in fiber can strengthen your immune system and help protect against these seasonal infections. By making mindful food and lifestyle choices, you can boost your resilience and stay healthier throughout the year. Remember, your gut microbiota have to be fed if you want them to be healthy.