**6. Immuno-modulatory effect of probiotics**

The gut of poultry serves as a critical interface where the immune system interacts with the external environment, including ingested feed and potential pathogens. This dynamic environment houses a diverse microbial community that plays a pivotal role in shaping the host's immune responses. Probiotics, as beneficial members of the gut microbiota, have gained attention for their immunomodulatory effects, influencing the intricate relationship between the gut and the immune system.

Certain probiotic strains have the ability to stimulate immune cells in the gutassociated lymphoid tissue (GALT), including dendritic cells, macrophages, and T cells. This activation enhances the recognition and response to pathogens while preventing excessive immune activation [48]. Moreover, Probiotics have been explored as potential adjuvants to enhance the efficacy of vaccines in poultry. By promoting a balanced immune response, probiotics can improve the recognition and memory formation of vaccine antigens. This leads to increased vaccine effectiveness and protection against pathogens. In a study conducted by Sarwar, it was observed that the vaccination against infectious bursal disease (IBDV) and Newcastle disease (ND) in combination with probiotic strains of *L. paracasei*, *L. casei*, *L acidophilus, Bifidobacterium*, *Streptococcus thermophiles* showed antibody titer improvement when compared control broiler groups administered with only with vaccine and probiotic alone [49].

Numerous studies have put forth the hypothesis that probiotics could potentially function as a viable substitute for antibiotics in the diets of poultry and are anticipated to boost animal immunity and health status. In a study, Penha Filho et al. presented that *Lactobacillus*-based probiotics proved to be effective against heavy infection of pathogenic enterobacterial infection of *Salmonella enteritidis* (SE) in chickens, by reducing SE's colonization in chicks. Study reported immunomodulatory effect of the probiotic such as, decrease in the levels of pro-inflammatory cytokines. It further showed stimulation of TLR2 expression in caecal tonsils which can further pay way to consider this probiotic as TLR2 based adjuvant with injectable vaccines [42]. On the other hand, a study observed upregulation of IFN-gamma mRNA genes in selenium enriched probiotic supplemented chickens when challenged with *E. Tenella*, thus providing protection against infection. Here the immunomodulatory response was linked with the increased antioxidant capacities [43]. Similarly, another research observed elevation of mRNA anti-inflammatory genes HIF1A (hypoxia inducible factor 1 alpha) and TSG-6 (tumor necrosis factor- (TNF) in the caecal mucosa of broilers fed with dietary probiotics when they were challenged with *P. multocida* [44]. *Lactobacillus fermentum* and *L. acidophilus* decrease the expression of pro-inflammatory cytokines in broilers affected with necrotic enteritis [45].

White pekin ducks' nonspecific immune responses were greatly boosted when their diets were supplemented with probiotics. Additionally, incorporation of probiotics to a low crude protein diet increased duck immunity to the same levels as those on a high crude protein diet [25]*.* Dietary addition of *L. plantarum* probiotic (1 × 108 CFU) showed significant (p < 0.05) improvement in both cell mediated and humoral immune response thus protecting the chickens against coccidiosis [46]*.* Additionally, rise in the levels of interleukin and immunoglobulin was observed by Memon et al. in broiler chicks thus explaining the synergistic activity of probiotic with synthetic drugs such as diclazuril, under induced Eimeria infection [47]. **Table 1** highlights the influence of probiotics on the immunity of poultry.
