Selenium (Se) forms part of selenoproteins, such as glutathione peroxidase, helping to reduce DNA and cell damage caused by reactive oxygen species (ROS). However, Se is also important for the immune system and influences gene expression (Hosnedlova et al., 2017).

1. Immune cells express many members of the selenoprotein family. Selenoproteins inside the immune cells perform antioxidant functions, carry out protein folding and promote specific cell signaling events during activation (Huang et al., 2012).
2. Macrophages and Neutrophils (innate immunity) produce nitric oxide and superoxide (ROS) during ingestion and elimination of pathogens (Kurutas, 2016).
3. Se is found to be critical to minimize the toxic effects of ROS, limit pathogen replication, and maintain immune cell function (Dalgaard et al., 2018).
4. Se is important for thymus development and influences the function and number of circulating T lymphocytes (acquired immunity) (Dalgaard et al., 2018; Arthur et al., 2003).
5. Se deficiency can lead to a depressed immune system resulting in increased susceptibility to disease. Se deficiency can also increase inflammation in numerous tissues, such as the gastrointestinal tract, the uterus, and the mammary gland (Avery and Hoffmann, 2018).
6. Spears (2000) found that 1mg Se/kg body weight at 21d before calving did not affect the incidence of clinical mastitis, but the duration of clinical symptoms was reduced by 46 %.
7. Hall et al. (2011) found the neutrophil function was suppressed in sheep affected with Foot Rot, while Se supplementation improved neutrophil function again.
8. Supplemental Se has been shown to increase disease resistance and/or alleviate symptoms associated with the disease in poultry, as observed for coccidiosis, Necrotic Enteritis, and E.coli (Dalgaard et a., 2018).
9. Liu et al. (2021) found that additional supplemented Se reduced proinflammatory cytokine and increased immunoglobulin concentrations in the serum under induced oxidative stress.
10. Lee et al. (2014) evaluated B Traxim Se on protective immunity against experimental avian necrotic enteritis in ovo. They found that Se increased intestinal levels of gene transcripts encoding proinflammatory and antioxidant genes resulting in enhanced protection against experimental necrotic enteritis.
11. Oxidative stress was also recently found to modulate the expression of genes in the DNA of sperm. These effects were reduced when males were supplemented with Se (Ferguson et al., 2012; Choucair et al., 2018).
12. Wyck et al. (2018) found that oxidative stress reduced motility and increased DNA damage to sperm. In addition to reduced viable sperm, altered gene expression negatively impacted early embryo development.

For Se to be biologically active to respond in the immune system and as an antioxidant, it must be incorporated into Selenoproteins (Sordillo, 2013). Thus, selenoproteins play a vital role in not only an antioxidant response but immune response and gene expression.