Introduction

Gut: The First Line of Immune Defence
The intestinal defence system consists of several interconnected components, including the mucus layer, intestinal epithelial cells, tight junctions, antimicrobial substances, gut microbiota and gut-associated lymphoid tissue (GALT).

The epithelial cells form a physical barrier between the intestinal contents and the internal tissues. Tight-junction proteins such as occludin, claudins and ZO-1 regulate the movement of substances between epithelial cells. The mucus layer provides an additional protective environment, while immune cells located within and beneath the intestinal mucosa continuously monitor microbial and dietary antigens.
Thus, the intestine acts as both a selective absorption system and an immune surveillance system.

Gut Microbiota and Immune Development
The intestinal tract contains a complex microbial ecosystem. A balanced microbial population can contribute to pathogen exclusion, production of microbial metabolites, maintenance of intestinal integrity and modulation of host immune responses.
Beneficial microorganisms and their metabolites communicate with intestinal epithelial cells and immune cells. Among these metabolites, short-chain fatty acids (SCFAs) such as acetate, propionate and butyrate are particularly important because they can influence epithelial health, immune-cell activity and inflammatory responses. Therefore, gut microbiota and immunity have a bidirectional relationship. A healthy microbiota supports a healthy intestinal barrier and balanced immune signalling. At the same time, the immune system regulates microbial populations and helps maintain intestinal homeostasis.
What Happens When Gut Health Is Compromised?
Commercial poultry are exposed to several factors capable of disturbing gut integrity, including coccidiosis, bacterial pathogens, mycotoxins, heat stress, poor water quality, nutritional imbalance and dysbiosis.

A compromised gut can result in epithelial damage, disruption of tight junctions, increased intestinal permeability, inflammation and oxidative stress, altered immune response and reduced performance. This process is sometimes described as increased “intestinal permeability” or “leaky gut.”

When intestinal integrity is compromised, nutrients may not be utilized efficiently and microbial products can interact more extensively with host immune mechanisms. The bird may consequently spend additional nutrients and metabolic resources on inflammation and tissue repair rather than growth or egg production.

Coccidiosis, Mycotoxins and Heat Stress

Coccidiosis is a classic example of the gut–immune relationship. Eimeria infection damages intestinal epithelial cells, alters intestinal morphology and stimulates an inflammatory and immune response. Consequently, coccidiosis control is not only a parasite-management issue but also a gut health, nutrition and immune-management issue.

Similarly, mycotoxins can affect intestinal integrity, oxidative status, microbial balance and immune function. Their impact may therefore extend beyond the feed itself to the overall gut–immune axis.

Heat stress is particularly important in tropical poultry production. Heat stress can alter feed intake, intestinal physiology, microbial balance and barrier integrity, potentially increasing inflammatory and oxidative stress. Maintaining gut integrity during heat stress is therefore an important component of maintaining flock resilience.

Gut Health and Vaccine Response
An effective vaccination program depends on the bird’s ability to recognize antigens and develop appropriate humoral and cellular immune responses. This requires adequate availability of energy, amino acids, vitamins, minerals and antioxidants. Therefore, gut health indirectly supports the nutritional foundation required for immune responses. A healthy intestine facilitates nutrient digestion and absorption, while intestinal inflammation or damage can increase nutrient requirements for tissue repair and immune activity. It is important, however, to recognize that good gut health alone does not guarantee vaccine success. Vaccine quality, administration, maternal immunity, biosecurity, disease pressure, nutrition and environmental conditions also influence the final immune response.

Nutritional Support for the Gut–Immune Axis
Nutrition is one of the most important tools for supporting the gut–immune relationship. Zinc contributes to epithelial integrity, cellular proliferation, enzyme activity and immune-cell function. Selenium is an important component of antioxidant defence systems and supports cellular protection and immune function. Vitamins A and E also contribute to mucosal integrity, antioxidant protection and immune responses.
Nutritional interventions such as probiotics, prebiotics, postbiotics, organic acids, enzymes, yeast-derived products, MOS and β-glucans may be used as part of integrated gut-health programs. MOS can support gut health through microbial interactions and pathogen-exclusion mechanisms, while β-glucans can interact with innate immune pathways and influence immune-cell activity. However, responses depend on product source, structure, dose, bird age, diet and challenge conditions; therefore, these products should be evaluated based on scientific evidence and field performance rather than assuming that all products have identical effects.

Practical Approach for Commercial Poultry
A successful gut-health program should not depend on a single feed additive. It should combine:
– High-quality and properly stored feed
– Regular mycotoxin monitoring and management
– Good drinking-water quality and sanitation
– Effective coccidiosis control
– Appropriate vaccination and biosecurity
– Heat-stress management
– Balanced amino acid, vitamin and mineral nutrition
– Appropriate use of probiotics, prebiotics/postbiotics and other gut-health products
– Monitoring of litter quality, feed intake, FCR, mortality and flock uniformity

Conclusion
The gut and immune system should be viewed as two interconnected components of the same biological defence network. A healthy intestinal microbiota, intact epithelial barrier and properly functioning GALT provide the foundation for balanced immune development and response. For commercial poultry, the concept is simple: “Protect the gut to support immunity, and support immunity to maintain flock resilience.”
Gut health is therefore not only about digestion and FCR. It is closely connected with nutrient utilization, immune development, disease resistance, vaccine responsiveness and ultimately flock productivity.
The future of poultry nutrition will increasingly focus on managing the gut–microbiota–immune axis through integrated nutrition, health and farm-management strategies rather than addressing individual problems in isolation.