It has been 12 years since the first proposal was made to sub- divide mouse CD4 I T cell clones into Th I and Th2 subsets, based on their differences in cytokine production, and 7 years since the first clear demonstration of a similar dichotomy among human T cell clones. In the ensuing period, it has been realized that inappropriate development of Th I or Th2 responses are important features of many immunological and infectious dis- eases. Perhaps the first group of diseases to be understood in terms of preferential Th subset activation were allergic diseases (see PARRONC'HI et aI. , this...