Memory CD4 T cells: generation, reactivation and re-assignment.

Immunological memory is one of the features that define the adaptive immune response: by generating specific memory cells after infection or vaccination, the host provides itself with a set of cells and molecules that can prevent future infections and disease. Despite the obvious importance of memory cells, memory CD4 T cells are incompletely understood. Here we discuss recent progress in understanding which activated T cells surmount the barrier to enter into the memory pool and, once generated, what signals are important for memory cell survival. There is still, however, little understanding of how (or even whether) memory CD4 T cells are useful once they have been created; a surprising thought considering the critical role CD4 T cells play in all adaptive primary immune responses. In light of this, we will discuss how CD4 T memory T cells respond to reactivation in vivo and whether they are malleable to a re-assignment of their effector response.