Specific combinations of AIM markers used to detect antigen-specific CD4+ in pertussis vaccine research to date include CD40L (also termed CD154) alone or co-expressed with CD69 and, most commonly, co-expression of OX40 plus CD25; the latter has been used to identify the maximal antigen-specific response, with or without PDL1 to discriminate antigen-responsive T-reg cells which upregulate CD25 upon antigen stimulation, a potentially important compartment that may contribute to underlying immunological differences between aP and wP vaccines, as discussed previously [152,155,156,157,158]. Here, CD4 is linked to pertussis.