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• Inverted T helper (CD4)/Cytotoxic T cell (CD8) ratio in Boran cattle. • Higher absolute numbers of circulating CD8 T cells in Boran cattle than EASZ, N'dama, Nelore and Holstein-Friesian. • Female cattle have significantly higher CD4/CD8 ratio than male cattle. The CD4+/CD8+ ratio is used as a marker of the immune regulation of T cell balance. When the ratio in peripheral blood is less than 1, this is considered an indication of immune suppression in an individual. Previous work on bovine Peripheral Blood Mononuclear Cells (PBMC) has consistently reported a ratio ≥1 as seen in other mammalian hosts, i.e. higher circulating CD4+ cell numbers than CD8+ cell numbers. However, a consistent inverted CD4+/CD8+ ratio (<1) was observed in Boran cattle, an African Bos indicus breed. The T cell populations were characterized in Boran cattle (n = 52), revealing higher percentages of circulating CD8+ cells (31.9 % average) than CD4+ cells (19.1 % average), thus resulting in the inversion of the expected T cell homeostasis in these animals. The results show that this inversion is not an effect of age or relatedness of the cattle, rather, it was shared by almost all Boran cattle used in this study. Despite this inversion being a feature shared by both males and females, the female cattle had significantly higher CD4+/CD8+ ratios than the male Boran. This paper describes the characteristics of the T cell fractions in the study animals and compares the findings to those of other Boran cattle in Kenya, and four other cattle breeds representing African indicine, African taurine, Brazilian indicine and European taurine cattle. We demonstrate that the consistent observation of inverted CD4+/CD8+ cell ratio was restricted to the Boran. [ABSTRACT FROM AUTHOR]