CD4 and infection: This time frame is probably too short for the majority of antigen-specific CD4+ T cells to reach peak frequency in a typical S. Typhimurium LAV infection but is more consistent with rapid activation of pre-existing, non-cognate CD4+ T cells, which respond to LPS stimulation and contribute to early infection control [45,46], coinciding with a critical period during which IFN-γ is required for antimicrobial control [12–14,16].