The pathogenic significance, if any, of these T cell abnormalitiesis still a matter of debate but there is a growing body of evidence that the entryof activated CD4 and CD8 T cells into the CNS and dysfunctional ‘cross talk’between the CNS and the peripheral immune system make a significant contributionto the genesis and/or exacerbation of pathology in at least some patients with AD[84, 85]. This evidence concerns the gene CD4 and Alzheimer disease.