**2.6 Th0 is the most dominant antigen-specific clone in cattle**

Just a few years after the discovery of the Th1/Th2 subtypes in humans and mice, Brown et al. successfully investigated bovine Th1/Th2 response through the establishment and analysis of antigen-specific CD4+ T cell clones. Peripheral blood mononuclear cells (PBMC) were purified from cattle challenged by experimental pathogens: either intracellular pathogens (*Babesia bovis*, *Babesia bigemina*) or extracellular pathogens (*Fasciola hepatica*) [22]. These purified PBMCs (that contained pathogen-specific CD4+ T cells), were stimulated with antigens derived from the same pathogen used for the challenge, to generate pathogen-specific CD4+ T cell clones, which were then analyzed and classified based on the detection of Th1/Th2 cytokine mRNA. The authors reported that, regardless of the type of pathogen used in the challenge, most bovine clones were Th0 that co-expressed IFNγ and IL-4 (*e.g.,* more than 60% *Babesia* species -specific and more than 90%

*Fasciola hepatica*-specific clones were Th0) [22]. These observations indicated that bovine Th1/Th2 responses might be at least partially different from the typical murine and human Th1/Th2 responses, as the frequency of bovine Th0 clones was significantly higher than that of murine and humans. Later, when researchers used the Th0 clones specific to an antigen of *Babesia bigemina* to stimulate B cells *in vitro*, both, Th1-related IgG2 and Th2-related IgG1 were detected in the supernatant culture, suggesting that Th0 is capable of performing functions of both Th1 and Th2 cells [138].
