Interestingly, previous infection with B. bronchiseptica (blue) did not increase the number of CD4+ T cells (Fig. 6A) producing IL-17 (Fig. 6D) in response to subsequent challenge with B. bronchiseptica; however, the numbers of CD4+ T cells increased in response to infection with B. pertussis when comparing with the sham vaccinated (Fig. 6B), with many of those being IL-17+ (Fig. 6E). Here, CD4 is linked to infection.