As much as 70–80% of immune cells are located in the gut!

Within the intestinal walls lies the so-called GALT (Gut-Associated Lymphoid Tissue) – the largest immune organ in the human body.

It is in the GALT that:

• viruses, bacteria, and toxins are identified,

• the body learns to tolerate “harmless” elements like food or beneficial bacteria,

• immune responses are initiated when threats are detected.

Gut microbiota – the immune system’s coach

Trillions of beneficial bacteria live in the intestines, acting as “coaches” for the immune system. They teach the body:

• what to attack and what to leave alone,

• how to respond proportionally (without exaggerated allergic or autoimmune reactions),

• how to maintain a balance between inflammation and resolution.

When the microbiota is weakened (e.g. due to antibiotics, poor diet, stress), the immune system may also become compromised.

The gut barrier – a protective wall

The intestinal wall functions like a filter: it allows beneficial substances (nutrients) in and blocks harmful ones (pathogens, toxins).
If this barrier becomes “leaky” (known as leaky gut), toxins and undigested particles can enter the bloodstream.
This activates the immune system, which may begin reacting to the body’s own tissues – potentially leading to autoimmune conditions.

The gut and systemic immunity

Healthy gut → balance throughout the body.
Unhealthy gut → chronic low-grade inflammation, weakened immunity, and greater susceptibility to infections.

The gut has a significant influence on hormonal balance, including those related to mood, appetite, digestion, and metabolism. Here’s how:

Serotonin – the “happiness hormone”

• 90–95% of serotonin is produced in the gut, not in the brain.

• It is synthesized by enterochromaffin cells in the intestinal lining.

• Serotonin affects gut motility (intestinal movement), mood and emotions (via the gut-brain axis), sleep, and appetite.

Ghrelin and leptin – the hunger and satiety hormones

• Ghrelin is produced in the stomach and acts on the gut and brain to increase appetite.

• Leptin is secreted by fat tissue but its signaling is closely linked to gut health.

• The state of the gut, including microbiota and inflammation, influences hunger perception, satiety, and body weight regulation.

GLP-1 and PYY – satiety and digestion hormones

• GLP-1 (glucagon-like peptide-1) is produced in the small intestine after eating.

  • Slows down gastric emptying,

  • Increases feelings of fullness,

  • Stimulates the pancreas to release insulin.

• PYY suppresses appetite and supports digestion.

Note: Weight-loss medications like semaglutide mimic the action of GLP-1.