Kindly check https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5930271/

Also check https://www.tandfonline.com/doi/full/10.1080/09712119.2016.1243119

Article Immunomodulatory effects of phytogenics in chickens and pigs

Environmental stressors like pathogens and toxins may depress the animal immune system through invasion of the gastrointestinal tract (GIT) tract, where they may impair performance and production, as well as lead to increased mortality rates. Therefore, protection of the GIT tract and improving animal health are top priorities in animal production. Being natural-sourced materials, phytochemicals are potential feed additives possessing multiple functions, including: anti-inflammatory, anti-fungal, anti-viral and antioxidative properties. This paper focuses on immunity-related physiological parameters regulated by phytochemicals, such as carvacrol, cinnamaldehyde, curcumin, and thymol; many studies have proven that these phytochemicals can improve animal performance and production. On the molecular level, the impact of inflammatory gene expression on underlying mechanisms was also examined, as were the effects of environmental stimuli and phytochemicals in initiating nuclear factor kappa B and mitogen-activated protein kinases signaling pathways and improving health conditions.

http://scholar.google.com/scholar?q=What+is+the+effect+of+phytogenic+on+the+immune+system+in+poultry%3F&hl=ar&as_sdt=0&as_vis=1&oi=scholart

Depend on the kind. A lot of. Please check aromatic plant.

Many phytogenic material can improve immune system of bird like moringa plant leave crushed or moringa seed,caniola seed .

النباتات الطبيةلها تاثير كبير في زيادة المناعة للطيور الداجنة وتحسين التستفادة من الغذاء وتحسين الهضم ضمن مستويات محددة وبدون اي تاثير جانبي على الصحة العامة والهضم .

Kindly check the following link:

https://scialert.net/abstract/?doi=ijps.2019.93.100

In which it had been reported that Biostrong 510 has the potential to stimulate the innate immune response and enhance antibody production against NDV.

Also, kindly check the following link:

https://scialert.net/fulltext/?doi=ajpsaj.2016.134.140

In which it had been reported that dried Cr and Tu feed additives had an immunomodulatory effect in broilers.