Immunostimulants are agents capable of exciting the vital energy [Brown1878; Meyer1932] to increase the activity of functional tissues [Sanchez BOTM680] to make an appropriate immune response. Note, however, that stimulating one aspect of the immune system tends to suppress other aspects and vice versa, as is seen in the TH1 vs TH2 response that is mediated by cytokines. For this reason, Immunomodulator seems to be a better term [ORWJr].
[Brown1878] Oliver Phelps Brown. The Complete Herbalist; or the People Their Own Physicians. Jersey City, NJ (1878). Online Reprint.
[Meyer1932] Joseph Ernest Meyer. The Herbalist and Herb Doctor. Hammond, IN: Indiana Botanic Gardens (1932) Online Reprint.
[Sanchez BOTM680] Joann Sanchez. Botanical Medicine 680 notes. Southwest College of Naturopathic Medicine.