REVIEW METHODOLOGY

The literature search encompassed publications from January 2000 to March 2025. No specific geographic restriction was applied; however, studies were drawn from global research to capture broad applicability across various poultry production systems.

This study was conducted as an integrative review to synthesize current evidence on the mechanistic roles of probiotics in modulating the gut–immune axis and their application as sustainable alternatives to AGPs in poultry production. An integrative approach was chosen to incorporate diverse study designs, including experimental, mechanistic, and applied research, thereby providing a comprehensive perspective on immune modulation and practical implications.

Literature search strategy

A structured literature search was performed using major scientific databases, including Scopus, Web of Science, and PubMed. The search employed combinations of keywords and Boolean operators, including “probiotics,” “poultry,” “gut–immune axis,” “immune modulation,” “intestinal morphology,” “vaccination response,” “oxidative stress,” and “antibiotic growth promoters” or “AGP replacement.” Reference lists of relevant reviews and primary studies were also screened to identify additional eligible articles.

Study selection and data synthesis

Studies were included if they (i) investigated probiotic supplementation in poultry species (e.g., broilers, layers, breeders, or indigenous chickens), (ii) reported immunological, microbiological, molecular, or production-related outcomes, and (iii) were original research articles published in English-language peer-reviewed journals. Studies focusing exclusively on non-poultry species, lacking measurable immune or gut health parameters, or published as conference abstracts and non-peer-reviewed reports were excluded.

Relevant data extracted from each study included probiotic strain(s), dosage, supplementation duration, experimental design, immune parameters (e.g., cytokine expression, antibody titers, lymphoid organ indices), gut morphology, microbiota composition, and production performance indicators. The findings were synthesized thematically to evaluate mechanistic pathways, consistency of outcomes, strain-specific effects, dose–response relationships, and translational relevance to commercial poultry systems, while also identifying key research gaps and industry implications.