Caffeine:
Various studies have demonstrated that caffeine can modulate different aspects of innate and adaptive immunity. Caffeine has been shown to affect cytokine production, free radical production, lymphocyte proliferation, antibody production, natural killer cell function, histamine release, and immune cell apoptosis (Horrigan et al. 2006). Myeloperoxidases are essential for the antimicrobial activity during neutrophil’s respiratory burst, and in vitro studies showed that caffeine significantly increases the release of myeloperoxidase from a mixed population of neutrophils and PBMC (Sullivan et al. 1995). As a reduction of neutrophils was observed in the peripheral blood of COVID-19 patients (Liu et al. 2020), the enhancement of myeloperoxidase activity by caffeine could potentially improve COVID-19 disease condition. In line with caffeine’s role in immunomodulation, an intriguing study reported that a bolus dose of 6 mg/kg caffeine caused an increase in total lymphocyte count and an increase in CD8+ T cell count (Bishop et al. 2005). The same study also reported that an increase of CD4+CD69+ T cells before and after exercise with pre-exercise caffeine ingestion only (Bishop et al. 2005). Therefore, it shows the immune stimulatory role of caffeine on T cells. Furthermore, a reduction in lymphocytes and CD8+ positive T cells in the peripheral blood of COVID-19 patients (Liu et al. 2020) means that caffeine may have the potential to increase the CD8+ T cells as well as the other cytotoxic CD4+CD69+ T cells in these patients and alleviate COVID-19 related symptoms by immune modulation