The intensification of poultry farming and the numerous accompanying stress factors lead to increased bird mortality and decreased production rates. Therefore, the main goal of this study was to verify the hypothesis that adding various phytobiotic compounds to poultry feed exerts immunomodulatory effects by altering selected cellular and humoral immune parameters, which may have implications for the health and productivity of the birds. The effect of using mixtures of polyphenolic compounds with tannins and essential oils (carvacrol, cinnamaldehyde, capsaicin) in the diet of Ross 308 broiler chickens on production rates and selected parameters of the cellular and humoral immune response was assessed. This goal was achieved by assessing the percentage of cells expressing CD3+/CD4+, CD3+/CD8α+, CD4+/CD25+, CD25+, MHC Class II+, Bu-1A+, CD79A+, TCRγδ+, and monocytes/macrophages in blood, spleen, and bursa of Fabrici tissue. The phytobiotic feed ingredients used in the study demonstrated various mechanisms of action on the birds' immune system. In the group of birds receiving the polyphenolic compound with tannin, an increase in the percentage of cells expressing MHC Class II in blood and spleen was observed, as well as an increase in the percentage of cells expressing CD4+CD25+ in blood. Medium Chain Fatty Acids (MCFA) and essential oils stimulated T lymphocyte activation (CD25) and MHC II expression, indicating increased antigen presentation and B and/or T cell activation. Polyphenols demonstrated strong immunoregulatory and anti-inflammatory effects while simultaneously stimulating cellular and humoral responses. Phytobiotics modulate the immune system of broilers, resulting in a beneficial effect on the health and productivity of the birds. Beneficial production effects, particularly in the broiler groups receiving MCFA and essential oils, were demonstrated, including higher body weight gain in the growing phase and lower FCR. Feeding broiler chickens with feed enriched with phytobiotics increases the growth potential of the birds and provides an effective, natural support for immunity and production performance.