In the present study, we identified a population of CD3+CD4-1+IFN-γ+ T cells (designated Th1 cells) in Nile tilapia using mAbs generated against CD3ε, CD4-1, and IFN-γ, and further investigated their immunological roles in resisting intracellular bacterial infection and the mechanism underlying their potential differentiation. This evidence concerns the gene ITGA2B and bacterial infectious disease.