The adaptive immune system declines with age for reasons that are partially structural. The slow rate of production of new immune cells in adults and the pace of turnover results in an effective cap on the number of these cells present at any one time. Immune cells are devoted to remembering threats as they occur, but some otherwise largely innocuous and widespread pathogens like cytomegalovirus cannot be effectively cleared from the body. Ever more memory T cells are devoted to that particular topic over the years, and this leaves ever less space for naive T cells capable of destroying invading pathogens. So to a first approximation the more memory cells you have in old age the worse off you are.
Here is a correlation between that measure of immune system dysregulation and age-related declines in cognitive function. We can speculate that linking mechanisms might include the chronic inflammation that accompanies age-related immune system dsyfunction or a decline in aspects of the supporting role played by portions of the immune system that are specific to brain tissue:
Immunosenescence and cognitive decline are common markers of the aging process. The current view is that the immune system plays a modulatory role in brain function, including in cognitive abilities and neurogenesis, which supports the notion that throughout life the brain is not "immune privileged" but rather "enjoys the privilege" of immune-dependent maintenance. Taking into consideration the heterogeneity observed in aging processes and the recently described link between lymphocytes and cognition, we herein explored the possibility of an association between alterations in lymphocytic populations and cognitive performance. In a cohort of cognitively healthy adults (n = 114), previously characterized by diverse neurocognitive/psychological performance patterns, detailed peripheral blood immunophenotyping of both the innate and adaptive immune systems was performed by flow cytometry.
Better cognitive performance was associated with lower numbers of effector memory CD4+ T cells and higher numbers of naive CD8+ T cells and B cells. Furthermore, effector memory CD4+ T cells were found to be predictors of general and executive function and memory, even when factors known to influence cognitive performance in older individuals (e.g., age, sex, education, and mood) were taken into account. This is the first study in humans associating specific phenotypes of the immune system with distinct cognitive performance in healthy aging.