There are over 3,000 different types in the plant genus which go by this name, but only one is relevant to traditional Chinese medicine... Astragalus propinquus (syn. Astragalus membranaceus). Known as Huang Qi, the root of this plant is believed to reinforce the underlying principle of qi, which means "life force" in that system. It is considered so important, they classify it as being one of the 50 fundamental herbs in their medicine.
What modern science tells us is that indeed, there is a high amount of antioxidants in astragalus root. Using the ORAC value methodology, about 75% of the antioxidant activity is detected using aqueous methanol extract, which would imply that astragalus is water soluble mostly, rather than fat soluble. How much antioxidants the root has does suggest there may be something to the immune boosting benefits long claimed by Chinese medicine. One study involving Astragalus membranaceus used extracorporeal shock-wave lithotripsy (ESWL) to intentionally cause oxidative kidney damage in rabbits (1). However the "renal morphological impairments were much milder" for the 15 rabbits treated with the root extract, when compared to the 15 who were not.
The suspected immunity benefits of this herb have been studied for an array of different diseases and conditions. Its flavonoids, saponins, and polysaccharides have been researched for possible anti-tumor activity related to liver cancer. Its anti-inflammatory effects might support the cardiovascular system. Some research even suggests it may lower blood pressure and act as a natural treatment for diabetes by increasing insulin sensitivity. While all of this is exciting, keep in mind that this research is all very preliminary and therefore, it is too early to conclude if these benefits truly exist.
Research Support, Non-U.S. Gov't: The Journal of Alternative and Complementary Medicine. Volume 13, Issue 1. NIH 2007