CD4 and infection: Mycoplasma turicensis” protection study, it was suggested that the dual expression of CD4+ and CD25+ might be used as a marker of regulatory T cells, which are able to suppress Th1 and Th2 responses, limiting the immune response to infection to prevent tissue damage and an excessive response (19, 39); however, CD4+ CD25+ expression alone is only an indirect marker of regulatory T cell populations (40); it is now known that Forkhead box P3 (FOXP3) should also be targeted to determine regulatory subsets (41), but such reagents were not available in this study.