"Tfh cell differentiation is a multistage, multifactorial process. There is no single event that defines Tfh cell differentiation, unlike T helper 1 (Th1) cell differentiation, for instance, which can be fully induced by interleukin-12 (IL-12) exposure in vitro or in vivo. Instead, Tfh cell differentiation is a multistep, multisignal process that also accommodates a significant amount of heterogeneity."