Traditional Chinese medicine has many different treatment methods, depending on the diagnosis. One of these principles is called tonification, where the natural functions and structures of the body are supported or enhanced. This can be especially important in a situation where someone may not have signs of illness, but they want to strengthen the natural immune response. One of the most important herbs for this is Huang Qi (Astragalus membranaceus). It strengthens what is known as the Wei Qi, or the aspect of the body that helps fight off external pathogens. This method is used while the person is still well, in order to enhance the immune response.
It is very important to remember that the tonification method should in most cases be stopped when the person starts to fight a cold or flu. Formulas appropriate to the presentation of the cold or flu should then be taken. Once symptoms are gone, the tonification method can be used again. In my clinical experience, people who ordinarily get frequent colds and flu begin to get them less and less often over time when using this method.
Some of the astgagalus-containing formulas that can be helpful, depending on the constitution of the patient:
Yu Ping Feng San (Jade Windscreen Powder)
Bu Fei Tang (Tonify the Lung Decoction)
Bu Zhong Yi Qi Tang (Strengthen the Middle and Augment Qi Decoction)
Shi Quan Da Bu Tang (All-Inclusive Great Tonifying Decoction)
There is some research on the ability of these formulas to enhance immune function, as well as research on astragalus on its own. Here is a monograph on the activity of astragalus: