Key Terms in Gut and Immune Health: A Reference Glossary

Microbiome

The collective community of microorganisms — bacteria, viruses, fungi, and other microbes — living in and on the human body. The gut microbiome, concentrated in the large intestine, is the most extensively studied subset and plays central roles in digestion, metabolism, and immune regulation.

Dysbiosis

An imbalance or disruption in the normal composition and diversity of the microbiome. Dysbiosis has been associated in research with a range of conditions including inflammatory bowel disease, metabolic disorders, and altered immune responses, though causality is still being established for many associations.

Probiotics

Live microorganisms that, when consumed in adequate amounts, may confer a health benefit on the host, according to the World Health Organization's widely cited definition. Common probiotic genera include Lactobacillus and Bifidobacterium, found in fermented foods and some dietary supplements.

Prebiotics

Non-digestible dietary components — primarily certain fibers — that selectively feed beneficial gut bacteria and promote their growth. Prebiotics are found naturally in foods such as garlic, onions, oats, and bananas.

Mucosal Immunity

The branch of the immune system that operates at mucosal surfaces — including the lining of the gut, respiratory tract, and urogenital tract. It represents the body's first major immunological barrier against pathogens entering through these routes.

Secretory IgA (sIgA)

The predominant antibody found in mucosal secretions such as saliva, breast milk, and intestinal fluid. Secretory IgA helps neutralize pathogens and toxins at mucosal surfaces before they can penetrate deeper tissues.

Tight Junctions

Protein complexes that seal the spaces between cells lining the intestinal wall, controlling what passes from the gut into the bloodstream. Disrupted tight junctions are associated with increased intestinal permeability, sometimes informally called 'leaky gut.'

Intestinal Permeability

A measure of how readily substances move across the intestinal lining into the bloodstream. When permeability increases beyond normal levels, larger molecules — including bacterial fragments — may cross, potentially triggering immune responses. Research into its clinical significance is ongoing.

Short-Chain Fatty Acids (SCFAs)

Compounds produced when gut bacteria ferment dietary fiber. The main SCFAs — acetate, propionate, and butyrate — serve as fuel for colon cells, influence gut motility, and have documented roles in immune regulation and inflammation modulation.

Innate Immunity

The immune system's rapid, non-specific first line of defense. It responds to pathogens through pattern recognition and physical barriers (like skin and gut mucosa) without requiring prior exposure to that specific threat.

Adaptive Immunity

The slower, highly specific arm of the immune system that generates targeted responses — including antibody production and T-cell activity — against previously encountered pathogens. Adaptive immunity is the basis of immunological memory and vaccine protection.

Inflammation

A normal immune response to injury, infection, or irritation, characterized by increased blood flow, immune cell recruitment, and chemical signaling. Chronic, low-grade inflammation — which can occur without obvious illness — is implicated in a wide range of noncommunicable diseases.