**2.5 Distinct Th1 and Th2 are the most dominant antigen-specific clones in mice and humans**

In mice and humans, Mosmann et al. and Romagnani et al. stimulated single CD4+ T cells *in vitro* and established antigen-specific CD4+ T cell clones, which they classified mostly into Th1 and Th2 subtypes. Although, in both mice and humans, clear-cut Th1 or Th2 were the dominant clones, a small percentage of hybrid clones (named "Th0" clones), that co-produced Th1 and Th2 cytokines (IFNγ and IL-4), were also observed [27, 28]. Subsequently, follow-up research verified the existence of these hybrid clones, which were only a small fraction of the total clones (*i.e.,* only 9.6% clones were Th0) [124, 129–135]. Therefore, at this moment, the consensus in the fields of murine and human immunology is that Th1 and Th2 are the major effector cells that orchestrate immune responses against intracellular and extracellular pathogens, respectively, and that Th0 are short-lived "intermediate" cells [131, 136, 137].
