Also known as purple coneflower, echinacea is a well-documented immune system stimulant which activates T and B lymphocytes. It also has marked antiviral properties. Echinacea enhances resistance, stimulates the lymphatic system, and is often used against colds and flu. It is best taken in small, frequent doses. If used on a long-term basis, take echinacea for only six days a week, or three weeks out of four, since it is more effective if not taken continuously.