PROBIOTICS AND ADAPTIVE IMMUNITY

Adaptive immunity in birds is a specific defense system that develops after exposure to an antigen, involving humoral and cellular responses [144]. The humoral response is characterized by antibody production by B cells, while the cellular response is driven by T lymphocytes, including CD4⁺ (T helper) and CD8⁺ (T cytotoxic). These two pathways work synergistically to enhance the bird’s ability to recognize and neutralize specific pathogens and to build immunological memory, providing long-term protection [145]. While improved adaptive immunity can enhance disease resistance, overstimulation of lymphocyte proliferation and sustained cytokine production may increase maintenance energy requirements, thereby potentially reducing growth rates or egg production under certain conditions.

Probiotic supplementation, such as Lactobacillus spp., Bifidobacterium spp., and Bacillus spp., has been shown to increase antibody titers against vaccines and pathogens in poultry, including NDV, IBDV, and Avian Influenza (AI). This suggests direct stimulation of B cell proliferation and differentiation into antibody-producing plasma cells, particularly IgA in the mucosa and IgY in the systemic circulation, thereby enhancing both local and systemic protection [146].

Nevertheless, responses differ substantially between broilers and layers. Broilers often show short-term increases in antibody titers during early growth phases, particularly when probiotics are administered before or at the time of primary vaccination. In contrast, layers, due to their longer production cycle, tend to exhibit more variable humoral responses, with some studies reporting stabilization rather than significant elevation of antibody titers [147].

Importantly, several in_ vivo_ studies have reported no significant improvement in NDV or IBDV antibody titers despite probiotic supplementation, with seroconversion levels comparable to non-supplemented controls [148, 149]. In some cases, increases were statistically significant but biologically marginal, without clear improvement in protection following challenge tests.

These inconsistent outcomes may be influenced by probiotic strain specificity, suboptimal dosage, duration of administration, vaccine type, environmental stressors, or the baseline immune status of the birds. In well-managed flocks with adequate nutrition and low pathogen pressure, additional probiotic supplementation may yield only marginal or statistically non-significant improvements in humoral responses [150].

Clear links to vaccination protocols have now been incorporated. Studies indicate that probiotic adminis-tration initiated 1–2 weeks prior to primary vaccination and continued through booster doses may enhance peak antibody titers, whereas supplementation started only after vaccination often results in limited or delayed effects [151]. Moreover, some trials reported enhanced early antibody response (e.g., at 7–14 days post-vaccination) without sustained differences at later time points, suggesting transient rather than durable immunomodulation [152].

Additionally, the immunological response to probiotic supplementation may differ between broilers and layers due to their distinct genetic selection goals and production physiology [153]. Broilers, which are selected for rapid growth and short production cycles, may exhibit more pronounced short-term improvements in antibody titers and growth-associated immune efficiency, but they may also be more sensitive to immune-related energy trade-offs [154]. In contrast, layers, characterized by longer production periods and sustained metabolic demands for egg production, may benefit more from long-term immune stabilization and balanced cytokine regulation rather than marked increases in antibody titers alone [155]. Consequently, probiotic strategies should consider production type (broiler vs. layer) to optimize both immune competence and performance outcomes.

In addition to enhancing B cell activity, probiotics also affect T lymphocyte populations, including increasing the number and activity of CD4⁺ and CD8⁺ cells [156]. CD4⁺ cells play a role in supporting B cell activation and coordinating the immune response, while CD8⁺ cells are responsible for targeting and destroying virus-infected or abnormal cells [157]. However, increases in CD4⁺ or CD8⁺ cell counts are not consistently associated with improved vaccine efficacy or disease resistance, highlighting that quantitative lymphocyte expansion does not always translate into functional immune superiority [158]. This modulation results in a more efficient cellular response to antigens while enhancing protection against pathogen infection.

Probiotics also influence the cytokine profile, which has immunomodulatory properties [159]. Studies have shown increased expression of IL-10, an anti-inflammatory cytokine that helps maintain a balanced immune response, and increased IFN-γ, which supports the activation of T cells and macrophages to eliminate pathogens [160]. Yet, contradictory findings exist: some studies report unchanged or even reduced IFN-γ expression following supplementation, particularly under non-challenge conditions [161]. This variability underscores the absence of a clear strain hierarchy and indicates that probiotic efficacy is context-dependent rather than universally positive. Nevertheless, shifts in cytokine expression should not be interpreted as inherently advanta-geous in all contexts [162]. For example, excessive IFN-γ production may intensify inflammatory responses, whereas elevated IL-10 levels could potentially dampen protective immunity if overexpressed [163].

Therefore, the immunological benefit of cytokine modulation depends on achieving an appropriate balance between pro-inflammatory and anti-inflammatory signals rather than simple upregulation of specific cytokines [164]. Taken together, current evidence indicates that probiotic effects on vaccine-induced humoral immunity are variable and context-dependent rather than universally positive [165]. This combined effect on cytokines, lymphocyte counts, and antibody production confirms that probiotics function as natural immunomodulators, enhancing the effectiveness and specificity of adaptive immunity in poultry [166]. Optimal poultry performance depends on maintaining immune competence without triggering unnecessary or prolonged immune activation.