After in vitro stimulation, a significantly higher number of CD4+ T cells obtained from all infected mice irrespective of age showed production of TNF-α, IFN-γ, IL-4, and IL-21 as compared to the cells from naïve uninfected mice demonstrating high proliferation of T cells as a response on infection with N40 and B. microti individually or together. This evidence concerns the gene CD4 and infection.