EFFECTS ON MAJOR IMMUNE ORGANS

Primary and secondary lymphoid organs in birds, such as the bursa of Fabricius, thymus, and spleen, play a crucial role in the formation and maturation of the immune system [167]. The bursa of Fabricius is a typical avian organ that serves as the site of B cell differentiation and maturation, while the thymus is responsible for the development of T lymphocytes [168]. The spleen, as a secondary lymphoid organ, plays a role in monitoring blood circulation, activating immune cells, and producing systemic antibodies [169].

Probiotic supplementation has been shown to influence the morphological development and relative weight of major lymphoid organs in poultry [170]. While some broiler trials report increased relative bursa or spleen weight following supplementation, others show no significant differences compared with controls [171]. Moreover, organ enlargement has not consistently correlated with improved antibody titers or enhanced protection following pathogen challenge. Studies have shown that administering probiotics, such as Lactobacillus spp., Bifidobacterium spp., and Bacillus spp., can increase the relative weight index of the bursa, thymus, and spleen, reflecting stimulation of immune cell proliferation and lymphoid tissue maturation [172]. This increase in relative weight is generally associated with increased lymphocyte counts and enlarged lymphoid follicles, indicating more immunologically active organs [173].

However, enlargement of lymphoid organs does not necessarily equate to improved immune competence. Increased organ weight may reflect transient immune activation, physiological adaptation, or even inflammatory responses rather than enhanced protective immunity [174]. Therefore, morphological enlargement should be interpreted alongside functional indicators, including antibody titers, cytokine profiles, pathogen challenge outcomes, and overall health performance, to accurately assess immunological benefits [175].

In addition to quantitative effects, probiotics also affect the histological structure of lymphoid organs [176]. The bursa of Fabricius showed increased lymphoid follicle diameter, cortical thickening, and a higher number of B cells, while the thymus experienced thymocyte proliferation and an increased cortex-to-medulla ratio [177]. In the spleen, probiotic supplementation increased lymphoid cell density in the white pulp and strengthened interactions between B and T cells [178]. These histological changes indicate that probiotics not only affect organ size but also enhance the functional ability of lymphoid organs to respond to antigens [179]. Nevertheless, sustained or excessive enlargement without corresponding improvements in immune efficiency may indicate immune overstimulation or increased metabolic burden, underscoring the need for balanced immune modulation rather than maximal organ growth.