The teachings provided herein generally relate to the preparation and uses of compositions comprising extracts of Astragalus membranaceus. The extracts are prepared from Astragalus membranaceus and can comprise, for example, an acid-modified arabinogalactan protein composition having an arabinose:galactose ratio ranging from about 3.5:1 to about 5.0:1, from about 5% to about 10% rhamnose, from about 15% to about 20% galactose, and from about 10% to about 15% glucose. The compositions can be used in the treatment of idiopathic thrombocytopenic purpura and the formulation of medicaments for such treatments.