Collectively, our results show for the first time, the likely impact of the TBX21 (T-bet) immune gene in late-onset Alzheimer’s’ disease development and that the elevated TBX21 transcripts in the leucocytes of peripheral blood in LOAD patients may represent a new easy diagnostic test for Alzheimer’s disease. This evidence concerns the gene TBX21 and early-onset autosomal dominant Alzheimer disease.