Not surprisingly, as we age, our immune systems falter. Older adults are more likely to succumb to infections their younger selves blithely warded off with barely a sniffle. Cancer rates increase exponentially with age, partly because of less effective tumor surveillance from an aging immune system. Dysfunctional immune cells cause chronic inflammation which increases the likelihood of degenerative diseases such as osteoporosis, atherosclerosis, and dementia. This condition, called ‘immunosenescence,’ results from changes in the relative proportions and function of subsets of white blood cells, the thymus, and the bone marrow.
One of the most prominent changes of immunosenescence is the decrease in the subset of white blood cells that are able to fight off new infections or tumors. Called naïve suppressor t-cells, they are designated by the lack of the CD95 and presence of the CD28 molecules protruding from the membranes of the cell’s surface. The presence of these CD95-CD28+ t-cells can be detected at specialized labs from the same of tube of blood used to do the routine CBC (complete blood count). Cells that do not have CD95 sticking out of them have not yet encountered the foreign substance (the molecular snippet of an infectious bug or tumor), called ‘antigen,’ they are uniquely created to recognize. The CD28 molecule enables these cells to divide rapidly and mount a brisk response to their designated antigen, which results in clearance of the infection/tumor.
The linear decline in the number of CD95-CD28+ t-cells is so highly correlated with age (r2 >0.5) that it has been called the best biomarker of immune aging discovered to date. When we are young adults, we have over 200-250 cells/mcl of blood but start to lose about 3 cell/mcl per year from then on. If you do the math, you can see that this leads to complete absence of the ability to fight off new infections and tumors by the ninth decade, which is just about the average life expectancy. If you add B-lymphocytes (CD19+) to the model with naïve t-cells, your understanding of age-related immune system changes increases even more (r2 >0.63).