Immunomodulatory effects of selected medicinal herbs and their essential oils: A comprehensive review

[Display omitted] •This review illustrates the immunomodulatory, anti-inflammatory, and antiviral effects of selected medicinal herbs that may help prevent COVID-19 infection and complications.•In experimental models, the eleven selected medicinal herbs and derived essential oils boost the immune system.•These herbs and derived essential oils are safe to consume at the recommended amounts.•Well-designed human clinical trials are needed to confirm the immunomodulatory effects of these herbs. Medicinal herbs and their essential oils are used in human health promotion and disease prevention since ancient times. In the last two decades, many studies have been carried out to both identify bioactive compounds in medicinal herbs and derived essential oils and to examine their biological effects in experimental models; clinical trials, however, have been scant. This review discusses in vitro, in vivo, and clinical evidence supporting the immunomodulatory role of eleven medicinal herbs (bay laurel, black cumin, clove, fennel, lemon balm, lemongrass, marjoram, peppermint, rosemary, sage, and thyme) and their essential oils and bioactive components. Safety and toxicity aspects for consumption as well as future perspectives are also covered. Relevant data from the existing literature have been compiled and summarized. These herbs and oils, which are increasingly consumed, can be considered as valuable dietary supplements due to their health-promoting bioactive constituents. Well-design clinical trials are warranted to better ascertain the immunomodulatory effects of these herbal products.