T cells, particularly CD4+ Th1 cells and NKT cells are major cellular subsets that produce IFN-γ and thus mediate protective immunity against Ehrlichia. To determine the role of NK cells in host defense against Ehrlichia during recall response, we examined the number of antigen-specific, IFN-γ-producing T cells and NKT cells in all mice groups on day 7 after the second IOE infection. The gene discussed is CD4; the disease is infection.