In the past, helminths were thought to simply suppress Th1 cells while inducing Th2 cells. Rook points out that this hypothesis would only explain the regulatory effects of parasitic worms on autoimmune diseases caused by Th1 cells. However, helminths also regulate Th2-caused diseases, such as allergy and asthma. Professor Rook postulates that different parasitic worms suppress different Th types, but always in favor of regulatory T (Treg) cells.
Source:
here.