Immune system is a main factor that influences the animal’s health and performance. It is well known that Se and Vit E can act synergistically and affect biological processes mainly, antioxidant and immunity . Dietary Vit E and Se combination improved the humoral immunity and provided better immunity response through their role on free radical elimination and oxidative stress stability of the immune cells  Generally, the results of this study indicated that there are synergistic effects between Vit E and Se on plasma immunoglobulin, which appeared just in plasma IgM level Table 4. Combination of Vit E with bacterial organic Se of ADS18 significantly increased plasma IgM concentration in broiler chickens compared with the chicks fed SS- Vit E complex or Vit E alone. This result was consistent with the finding that Vit E and Se combination improved the humoral immunity and provided better immunity response [8, 11], when a dietary vitamin E (0, 125 and 250 mg/kg), selenium (Se, 0, 0.5 and 1 mg/kg), and their different combinations on humoral immunity was examined by intravenous injection of 7% sheep red blood cell (SRBC) followed by evaluation of serum for antibody titers in primary and secondary responses. Vitamin E and Se showed interactive effects on antiSRBC titers. Dietary vitamin E and Se alone also resulted in improvement of primary and secondary antibody responses . On the other hand, Supplementation of 200 mg vitamin E/kg and 0·2 mg selenium/kg resulted in a significantly higher antibody titres, that associated with an increased serum concentration of total immunoglobulins and circulatory immune complexes . Moreover, combined Vit E and Se, significantly increased serum IgG and IgM levels in mice after their exposure to sodium azide  .
Dietary supplementation of Vit E in broiler chicken improved primary and secondary antibody responses after heat stress challenge . According to Habibian et al. , supplementation of Vit E at both 125 and 250 mg/kg levels showed no effect on IgM level in broiler chickens. Furthermore, Vit E supplementation had no significant effect on immunoglobulin levels of IgA, IgM, and IgG in the serum of duck . This partially agreed with this study’s finding that there was no significant effect observed in IgA and IgG levels due to Vit E supplementation, however IgM level was significantly increased at day 21, but at day 42 the effect was significantly interactive with Se factor.
On the other hand, Khan et al. , demonstrated that dietary Se (inorganic Se) could improve immunity and significantly enhance the synthesis of IgA and IgG in broilers. This was clearly observed in this study, that Se supplementation of both inorganic and bacterial organic sources improved the bird’s immunity via IgA, IgG, and IgM elevation at day 42, but at day 21 bacterial organic Se showed significantly highest IgG level compared to SS and NS groups Table 5. However, our study concern about the different Se sources mainly the bacterial organic Se as unusual source of Se. This could be explained by the role of Se in protection and thus activation of B-lymphocytes cells which is the source of immunoglobulin . Moreover, Se could increase the interleukin 2 receptors on the surface of lymphocytes .
The regulation of gut microbiota can be achieved via dietary supplements which have the ability to stimulate the growth of beneficial bacteria, or selectively can suppress the activity and growth of pathogenic bacteria. Trace elements as a diet component may affect the diversity of intestinal microbiota . A study conducted by Molan et al. , revealed that the addition of sodium selenite or sodium selenate alone in the concentration of (1, 2, 3, 4, and 5 μg/ml) for each source, or their combination with China green tea to MRS culture of Lactobacillus rhamnosus and Bifidobacterium breve, enhanced their growth significantly compared to the control MRS. Oral supplementation of Se- china tea extract significantly (P < 0.05) increased the count of both Lactobacillus spp. and Bifidobacterium spp. in rat’s ceacum compared to tea extract without Se . Moreover, bacterial culture of Lactobacillus casei rhamnosus showed improved cell viability when Se and cadmium (Cd) were added to the culture compared to the culture contained in the Cd alone . These studies supported the finding of the current study that Se supplementation enhances the population of ceacum Lactobacillus spp. and Bifidobacterium spp. compared to the basal diet, but the Se sourced from ADS18 showed better count than SS treatment.
Furthermore, the current study showed that Se supplementation of inorganic and bacterial organic Se reduced the ceacum numbers of E-coli and Salmonella spp., but the difference was significant just between ADS18-Se and NS groups. In the same manner, feeding of probiotics or Se-enriched probiotics to piglets was associated with higher Lactobacillus spp. count and lower E-coli compared to the basal diet and sodium selenite diet . Selenium-enriched probiotics also decreased E-coli via undefined antimicrobial metabolites . In contrast, dietary Se-yeast in chickens showed no effect on the caecal microbiota or Campylobacter jejuni colonisation . Intestinal microbiota, are probably sensitive to some trace elements such as Se, which is important for normal function for some bacteria, however, at the same time is considered a toxic element to other bacteria. Therefore, changes in dietary Se which acts as an antioxidant may modulate the diversity of intestinal microbiota via oxidative stress suppression and providing a better medium for the growth of beneficial bacteria. Lactic acids and Bifidobacterium spp. are able to incorporate Se from the growth medium to their cells [26, 30], which may enhance their growth and activity. Moreover, lactic acid bacteria have a role in inhibiting the colonization of pathogenic microorganisms through their secreted hydrogen peroxide, acids, and other antimicrobial substances. Published data on the effect of Vit E on caecum microorganisms are scant, thus, the reduction in Salmonella spp. count in the present study due to Vit E supplementation remain unclear.
Cytokines are small protein messengers released by the host as immune responses to infection, inflammation, and trauma. Some cytokines are anti-inflammatory and others are pro-inflammatory, and this also plays a role in the up-regulation of inflammatory reactions . TNF-α, IFN-γ, and IL2 are pro-inflammatory cytokines that are produced by activated monocytes, macrophages, T cells, and natural killer cells. Adequate Se supplementation may enhance the immunity and decrease the host susceptibility to diseases . Infected Se- deficient mice were associated with significant increases in the expression of cytokines IL-6, TNF-α, IFN-γ IL-18, and IL-10 in the liver, while Se adequate mice showed over-expression in the liver IL-10 . However, in the present study, there was significant interaction between Se and Vit E on all examined splenic anti-inflammatory and pro-inflammatory cytokines expression, dietary Se of both ADS18 and SS down-regulated the IFN-γ and up-regulated IL-2 and IL-10 significantly compared to NS group, while ADS18 showed significant down-regulation of TNF-α level compared to both SS and NS groups. The down-regulation of pro-inflammatory cytokines observed in this study was due to IL-10 elevation, which is a known potent anti-inflammatory cytokine and deactivator of pro-inflammatory cytokine synthesis, macrophage, and monocyte . However, the IL-2 up-regulation in this study was unexpected but according to Yang et al. , IL-2, is also required for the growth, proliferation, and differentiation of T cells, and the spleen sight is highly enriched with T cells, B cells, and monocytes.
On the other hand, Vit E supplementation down-regulated the pro-inflammatory cytokines expression in chickens receiving lipopolysaccharide . The present study indicated that combination of either SS or ADS18-Se with Vit E had no significant effect on IFN- γ and IL-10 compared to Vit E alone, while Vit E alone showed the significantly lowest TNF-αcompared to both Se combinations. To the best of the researcher‘s knowledge, there is no report about the effect of Se and Vit E combination on splenic cytokines expression, however, previous studies indicated that dietary Vit E could lower pro-inflammatory cytokine expression through the NF-κB pathway alteration . On the other hand, dietary Se-regulated inflammatory cytokines via NF-κB and MAPK signaling pathways . Therefore, it is clear that, Se can induce immune response more than Vit E, through activation of anti-inflammatory cytokines and suppression of pro-inflammatory cytokines. The balance between anti-inflammatory and pro-inflammatory cytokines determines the severity of the disease.
In the current study, there was improvement in the final body weight and ADFI due to Vit E supplementation, and due to dietary Se either in bacterial organic or inorganic form. This was proved by the finding that supplementation of organic selenium and Vit E in the layer diet was efficient for improving performance . Supplementation of 200 mg/kg of Vit E in broiler diet improved weight gain and feed intake compared to the basal diet . In contrast, dietary Vit E and Se had no effect on the body weight of layer hens , and different Vit E levels showed no significant changes in the broiler performance . On the other hand, present study showed that both Se and Vit E supplementation had no significant effect on the lymphoid organ weights (thymus, bursa, and spleen) in broiler chickens. These findings are consistent with previous results of  who revealed that Vit E supplementation could not affect the lymphoid organ. Moreover, Habibian et al.  indicated that dietary Se had no positive effect on the lymphoid organ weights under heat stress, however, the dietary inclusion of Vit E showed improvement of the relative weights of lymphoid organs. Along the same lines of the findings of this study, no synergestic effect between Se and Vit E were observed for relative lymphoid organ weights in the study conducted by Habibian et al. , however, Swain et al.  reported that dietary Se and Vit E had a synergestic effect on the lymphoid organ weights in broiler chickens under normal environmental conditions.The reason for this difference is not clear.