In 1989, Strachan proposed the hygiene hypothesis, observing that children from larger families, presumably with more microbial exposure, were less likely to develop hay fever (1). This landmark idea reshaped how researchers viewed the relationship between microbes and immunity.

The concept later evolved into the old friends hypothesis. This model proposed that the rise in allergy and autoimmunity was driven not by reduced infection exposure, but by the loss of evolutionarily ancient microbes—commensal and environmental organisms that had long coexisted with humans. (2).

Later, the immune education model highlighted the importance of microbial interactions in shaping the developing immune system during infancy (3). Gut colonization was seen as essential for “training” T cells, establishing tolerance, and preventing hypersensitivity.

These models were groundbreaking, but all share a central assumption: that the immune system matures into independence after early microbial input. Once educated, tolerance was thought to persist even in the absence of continuous microbial influence.

However, mounting evidence suggests the opposite: the immune system requires ongoing microbial restraint throughout life (4, 5). Without continuous peace signals from microbiota, the immune system defaults to a heightened state of vigilance, prone to misfiring against pollen, food antigens, or self-tissues. This reframing moves beyond hygiene and education, recognizing microbes not as transient tutors but as lifelong partners in immune regulation.

Short-chain fatty acids (SCFAs)—acetate, propionate, and butyrate—are the key microbial signals that restrain immunity. Produced via anaerobic fermentation of dietary fibers, amino acids, and lipids, they: (i) promote regulatory T cell differentiation through HDAC inhibition and epigenetic programming (6); (ii) suppress Th2 and Th17 effector responses that underlie allergy and autoimmunity (6); (iii) inhibit mast cell degranulation (7); and (iv) enhance epithelial barrier function, reducing inappropriate antigen entry (8, 34). Other microbially derived metabolites (such as tryptophan catabolites or bile-acid derivatives) may also modulate immune tone, but current evidence for them as broad, cross-tissue “peace signals” is more limited and remains an area for future investigation (35, 36).

SCFAs are unique because they are metabolized rapidly by colonocytes, hepatocytes, and peripheral tissues (9, 10). They are oxidized for energy, giving them a short half-life. Unlike vitamins or structural microbial components, they cannot be stored. Their signaling value lies in their transience: SCFAs are only present when microbial fermenters are metabolically active.

In other words, immune restraint depends not on past microbial encounters but on continuous microbial peacekeeping in the present moment.

During infection, however, this peacekeeping is deliberately suspended. Microbial activity and SCFA levels decline due to fever, diarrhea during acute infection (11, 12), releasing the immune system from restraint. This allows vigorous responses to eliminate pathogens. Such reversible cancellation underscores the adaptive nature of SCFA signaling: peace in health, but war during infection.

The Microbial Peace-Signal Hypothesis emphasizes that SCFAs are not static background signals but dynamic modulators of immune restraint. Their production and function vary across biological contexts (Figure 1).

• Health: Balanced SCFA output maintains tolerance, with regulatory T cells, mast cell suppression, and strengthened epithelial barriers ensuring immune calm.

• Allergy and autoimmunity: Loss or decline of SCFA-producing guilds reduces peace signals, leaving the immune system unrestrained and prone to overreaction against pollen, foods, or self-tissues.

• Infection: SCFA production may be intentionally suppressed during infection (11, 12), canceling peace signals to allow the immune system to fight without restraint. This enables vigorous mechanisms to flush pathogens.

• Over-pacification: Excessive SCFA signaling may over-suppress immunity, increasing vulnerability to pathogens. This may explain why many classical SCFA producers (e.g., Fusobacterium, Peptostreptococcus) were first identified in infectious pus (13, 14). Here we use “over-pacification” to describe such states of local immune hyporesponsiveness driven by sustained, high SCFA exposure, which can be exploited by pathogens even though the same mechanisms are beneficial in health.

The gut microbiota remains the most studied SCFA system. Clostridia clusters IV and XIVa induce colonic regulatory T cells (15), Faecalibacterium prausnitzii suppresses pro-inflammatory cytokines (16), and dietary fiber intake correlates with reduced asthma and eczema incidence (17). High-SCFA diets protect against allergic inflammation in animal models, and human cohort studies consistently link low fecal SCFAs to allergy and inflammatory bowel disease (15–17).

Yet the gut is not the only “peace hub,” a term we use for anatomical sites where commensal fermenters generate SCFAs that locally restrain immunity (Figure 2). Two other sites—oral and skin microbiota—harbor dense SCFA-producing guilds with potential systemic reach.

Why, then, are allergies and autoimmune diseases epidemic in industrialized societies? The Microbial Peace-Signal Hypothesis argues that the root cause is the collapse of microbial niches that sustain SCFA producers.

• Oral hygiene: Aggressive brushing, mouthwash, tongue scraping, and antiseptics disrupt anaerobic guilds in gum pockets and tongue biofilms.

• Skin hygiene: Frequent washing, soaps, and alcohol-based sanitizers strip sebum and commensals.

• Antibiotics: Broad-spectrum treatments indiscriminately eliminate anaerobic fermenters in gut, mouth, and skin.

• Diet: Low-fiber, processed diets deprive gut and oral fermenters of substrates.

• Interruption of early seeding in newborns: Cesarean delivery, reduced breastfeeding, limited skin-to-skin contact, and confinement in highly sanitized hospital environments delay or diminish the transfer of maternal microbiota to infants, weakening the establishment of oral, skin, and gut SCFA guilds during a critical window of immune calibration (29–31).

These pressures collectively erode the ecological homes of SCFA producers. Without continuous peace signals, the immune system reverts to hyper-reactivity, attacking pollen, foods, or self-tissues.

The hypothesis makes specific, testable predictions.

The hygiene hypothesis reframed infection avoidance as a risk factor. The old friend hypothesis emphasized co-evolutionary microbial partners. The immune education model highlighted infancy as a critical window.

The Microbial Peace-Signal Hypothesis extends these by asserting that: (i) immune restraint is not a one-time event but a lifelong microbial function; (ii) SCFAs serve as dynamic peace signals, continuously consumed, continuously renewed; (iii) guild redundancy across gut, mouth, and skin is essential, ensuring resilience but also explaining differential susceptibility; and (iv) infection control requires temporary suspension of peace signals, while microbial misuse can flip tolerance into persistence. This reframing unifies allergy and autoimmunity under a single principle: both are consequences of collapsed microbial peacekeeping systems.

Immune homeostasis is not a static legacy of early-life microbial encounters but a continuous partnership with commensal fermenters. SCFAs act as biochemical peace signals, transient yet powerful, restraining immunity in real time. When the guild of SCFA producers thrives—in gut, mouth, and skin—the immune system remains calm. When modern life erodes these guilds, peace signals fall silent, and immunity overreacts. The Microbial Peace-Signal Hypothesis reframes allergy and autoimmunity as failures of microbial–immune symbiosis. It highlights diagnostics, lifestyle practices, and therapeutic strategies that restore microbial peace as the next frontier in immunology.