MUCOSAL IMMUNITY (GALT)

Mucosal immunity in birds, collectively referred to as GALT, is a crucial component of the avian defense system and serves as the first line of defense against pathogens entering the gastrointestinal tract [81]. Probiotics interact directly with GALT by influencing antigen sampling in Peyer’s patches and cecal tonsils, where microbial-associated molecular patterns (MAMPs) derived from probiotic strains are recognized by PRRs expressed on epithelial and immune cells. Through these interactions, GALT helps maintain immune homeostasis by integrating signals from microbes, nutrients, and environmental stimuli to generate effective yet controlled immune responses [82]. Rather than describing GALT solely as a structural immune compartment, this section emphasizes its functional role in mediating probiotic-driven immunomodulation, particularly through MAMP–PRR interactions that influence antigen presentation and stimulate IgA production in Peyer’s patches and cecal tonsils [83]. A summary of the principal components of the avian immune system, including GALT, innate immunity, and adaptive immunity, together with their main structures or cells and corresponding functions, is presented in Table 3[84–135].

Peyer’s patches, scattered throughout the small intestine, act as immune surveillance centers, involving M cells and various immune cells, including B lymphocytes, T lymphocytes, and dendritic cells [84]. Probiotic bacteria can be translocated by M cells to underlying dendritic cells, promoting antigen presentation and stimulating B cell differentiation into IgA-secreting plasma cells. Enhanced mucosal IgA production is one of the most consistently reported immune outcomes of Lactobacillus and selected Bacillus strains in poultry. IgA limits pathogen adhesion and neutralizes toxins at the epithelial surface [85]. IgA provides local protection by neutralizing pathogens and preventing microorganisms from adhering to the intestinal epithelium [86].

The bursa of Fabricius, a unique lymphoid organ in birds, serves as the primary site of B cell maturation and differentiation [87]. This organ not only plays a role in the production of diverse antibodies but also interacts with various antigens originating from the gastrointestinal tract, thereby supporting the emergence of humoral immune responses at both systemic and mucosal levels [88]. The role of the bursa of Fabricius is crucial, especially in the early stages of a bird’s life when the immune system is still developing [89].

The pharyngeal and cecal tonsils serve as antigen detection centers in the proximal and distal parts of the digestive tract [90]. The cecal tonsils, located at the base of the cecum, are one of the largest lymphoid tissues in birds and play a crucial role in processing signals from gut microbes [91]. Probiotic-mediated modulation of microbiota composition influences cytokine production within these tissues, promoting balanced Th1/Th2 responses and supporting vaccine-induced immunity. These structures are populated by T and B lymphocytes and antigen-presenting cells, which play a role in regulating inflammatory responses and maintaining immune tolerance to commensal microbiota [92].