Th1-and Th2-type cytokines (termed Th0) have been likely that the dose and route of immunization of antigen described in human and mouse systems (Sher and Coffmay also be determining factors (Seder and Paul, 1994; man, 1992; Romagnani, 1994; Kelso, 1995; Abbas et al., Abbas et al., 1996; Constant and Bottomly, 1997). 1996). Whether Th0 cells are precursors for Th1 and

Th2 cells or represent a separate, stably differentiated Heterogeneity of T Cell Populations population remains unclear (Kamogawa et al., 1993). It Th1 cells, by their production of interferon-(IFN) and is possible that Th0 cells are involved in eliminating lymphotoxin, areresponsiblefordirectingcell-mediated many pathogens, where a balance of both regulated immune responses leading to the eradication of intracel- cell-mediated immunity and an appropriate humoral relular pathogens (Mosmann et al., 1986; Sher and Coff- sponse will eradicate an invading pathogen with miniman, 1992), but they may also cause immunopathology mum immunopathology. However, chronic conditions and organ-specific autoimmune disease if dysregulated may result in polarized Th1-and Th2-type responses,(O’Garra and Murphy, 1993; Powrie and Coffman, 1993; which, by the counter-regulatory effects of the cytokines made by the reciprocal subsets, may not only be mutu-Scott et al., 1994; Liblau et al., 1995). Because cytokines ally exclusive but in addition pathogenic (Alwan et al., produced by Th2 cells, such as interleukin (IL)-4 and 1994). Towhat numericalextent Th1and Th2 cellsdomi-IL-5, can activate mast cells and eosinophils and in nate such in vivo responses is as yet not clear, but their