A recent study by Chen et al. published in Nature reported the presence of activated T cells in the brains of transgenic mice with frontotemporal dementia (FTD)-like Tau protein pathology and in postmortem Alzheimer’s disease (AD) brains.1 The infiltration of T cells correlated with the degree of Tau pathology and microglia activation, and appeared to promote neurotoxicity. The gene discussed is MAPT; the disease is Alzheimer disease.