PROBIOTICS IN INCREASING INNATE IMMUNITY

Innate immunity in birds serves as the initial defense against infection and involves various phagocytic cells, including macrophages, heterophils, and antigen-presenting cells such as dendritic cells. These components play a crucial role in recognizing and inhibiting pathogen development before the adaptive immune system kicks in [137]. Macrophage activation is a key process in the innate response, enhancing phagocytic capacity and the production of early inflammatory mediators [138]. Table 4 summarizes the effects of probiotics on the avian immune system, including innate and adaptive immune responses, lymphoid organ function, mechanisms for reducing stress and inflammation, and changes in gut microbiota composition [138–200].

Immune domain | Components/organs | Main probiotic mechanisms | Functional impact on poultry | Key considerations/limitations | References
Innate immunity | Macrophages | Enhance phagocytosis, stimulate early inflammatory mediator production, and activate NF-κB signaling through pattern recognition receptor-mediated pathways | Improve pathogen recognition, phagocytic efficiency, and early innate immune responses | Responses are strain-, dose-, and challenge-dependent; excessive activation may increase metabolic demands and oxidative stress | [138, 139]
 | Heterophils | Enhance degranulation and respiratory burst activity | Improve rapid elimination of bacterial and opportunistic pathogens | Responses vary among probiotic strains and environmental conditions | [140]
 | Dendritic cells | Upregulate co-stimulatory molecules and enhance antigen presentation | Promote activation of T and B lymphocytes and initiation of adaptive immune responses | Effects are generally more pronounced under pathogen challenge than under low-pathogen conditions | [141, 142]
 | Natural killer (NK) cells | Enhance cytotoxic signaling and metabolite-mediated activation | Increase elimination of virus-infected and abnormal cells | Functional evidence in poultry remains relatively limited | [143–145]
Adaptive immunity | B cells/plasma cells | Promote proliferation and differentiation into IgA- and IgY-producing plasma cells | Enhance mucosal and systemic immunity and improve post-vaccination antibody responses (e.g., NDV, IBDV, and AI) | Responses vary according to production type, vaccination protocol, and probiotic strain | [146–155]
 | T lymphocytes (CD4⁺ and CD8⁺) | Promote proliferation, activation, and cytokine-mediated immune coordination | Improve coordination of immune responses and elimination of infected cells | Increased T cell numbers do not always correspond to enhanced protective immunity | [156–158]
 | Immunomodulatory cytokines | Modulate expression of IL-10, IFN-γ, IL-1β, IL-6, and TNF-α | Maintain a balanced pro- and anti-inflammatory immune response | Benefits are context-dependent, and excessive cytokine expression may be detrimental | [159–165]
Major immune organs | Bursa of Fabricius | Increase follicular diameter, B cell proliferation, and relative organ weight | Promote B cell maturation and enhance humoral immune competence | Morphological changes should be interpreted together with functional immune indicators | [166–176]
 | Thymus | Increase thymocyte proliferation and cortex-to-medulla ratio | Promote T cell maturation and strengthen cellular immunity | Histological changes do not always indicate improved immune protection | [177]
 | Spleen | Increase lymphoid cell density in the white pulp and enhance B- and T cell interactions | Strengthen systemic immune activation and antibody production | Increased organ weight alone may reflect transient immune activation rather than functional improvement | [178, 179]
Stress and inflammation | Heterophil-to-lymphocyte ratio and corticosterone | Reduce physiological stress indicators and inflammatory signaling | Decrease physiological stress and excessive inflammatory responses | Benefits are more evident under heat stress, high stocking density, or infectious challenge | [180–184]
 | Reactive oxygen species and oxidative stress markers | Increase antioxidant enzyme activity (superoxide dismutase (SOD), GPx, and catalase) and reduce malondialdehyde and reactive oxygen species levels | Reduce oxidative damage and improve physiological resilience | Antioxidant effects depend on baseline oxidative stress and management conditions | [185–190]
Gut microbiota and gut–immune axis | Gut microbiota composition and microbial metabolites | Increase beneficial bacteria (Lactobacillus and Bifidobacterium), promote competitive exclusion of pathogens, and enhance short-chain fatty acid production | Strengthen epithelial barrier function, maintain immune homeostasis, and reduce pathogen colonization | Changes in microbial composition do not always correlate directly with measurable improvements in immune function | [191–200]

Probiotic supplementation with Lactobacillus spp., Bifidobacterium spp., and Bacillus spp. has been shown to stimulate macrophage activation through interactions with receptors on the surface of immune cells. This activation initiates immune signaling pathways, including nuclear factor kappa B (NF-κB), which enhances macrophages’ ability to ingest and destroy pathogens [139]. Nevertheless, excessive activation of inflammatory signaling pathways may increase metabolic costs and oxidative stress, which can negatively affect growth performance if not properly regulated. However, much of the mechanistic evidence for macrophage activation originates from in vitro cell culture studies or controlled experimental challenge models. In commercial in_ vivo_ settings, improvements in phagocytic activity or cytokine expression are sometimes modest or statistically non-significant, particularly in flocks with low pathogen pressure [140]. Furthermore, probiotics also enhance phagocytic activity, making immune cells more efficient in eliminating bacteria, viruses, and opportunistic microbes that invade the poultry digestive tract.

In addition to enhancing immune cell activity, probiotics regulate early-phase cytokine expression [141]. Research shows that probiotic supplementation can stimulate the production of pro-inflammatory cytokines, such as IL-1β, IL-6, and TNF-α, which function to coordinate the early immune response, attract additional immune cells, and strengthen mucosal defenses without causing chronic inflammation [142]. However, the induction of these cytokines is not universally beneficial; excessive or prolonged upregulation may contribute to tissue damage or chronic inflammation, depending on the host condition, pathogen challenge, and probiotic strain used. Notably, cytokine modulation appears to be strain-dependent, and dose–response relationships are not always linear. Several trials have reported minimal changes in innate immune gene expression with low probiotic doses, whereas excessively high doses did not proportionally enhance immune parameters and, in some cases, increased inflammatory markers without performance benefits [143].