Memory. is the diary that we all carry about with us. Oscar Wilde, ‘‘The Importance of Being Earnest’’ And so it is with the immune system. Memory T cells are a critical component of the adaptive immune system. Our ability to respond rapidly to antigens/ pathogens that we have been exposed to previously is at the heart of the adaptive immune system. In general, these memory T cells allow protection of the host to pathogens that the individual has previously encountered and/or been vaccinated against. The secondary response is rapid and more potent, leading to a rapid clearance of the offending agent. Not surprisingly, this population has also been explored in their contribution to graft-versus-host disease (GVHD).