How does your immune system age at the cellular level, and can a blood test predict that?
Your immune system ages in recognisable ways: exhausted immune cells and smouldering inflammation are clearly visible features. The markers on which a blood test would be based are already well described, though no such standardised test yet exists.
The immune system ages along two interconnected lines. First, the thymus, the organ that produces new immune cells, shrinks considerably with age. As a result, fewer and fewer fresh, untrained immune cells enter the bloodstream. Second, the existing immune cells become exhausted: they lose an activation protein on their surface and instead acquire more inhibitory proteins, which makes them less capable of responding to new threats. This pattern has been found consistently across multiple reviews and larger analyses.
A direct consequence of this is inflammaging: aged and exhausted cells secrete inflammatory substances that cause a smouldering, persistent inflammation. This low-grade inflammation is associated with a higher risk of cardiovascular disease, cancer and brain disorders later in life. Another concrete consequence is that older people respond less well to vaccines, as has been demonstrated repeatedly with the influenza vaccine. These are two well-documented practical consequences of immunosenescence.
In principle, aged immune cells can be measured via a blood test: a blood sample can be examined for the proportion of exhausted cells, the quantity of inhibitory proteins on the cell surface and the ratio between fresh and 'used' immune cells. However, researchers from two systematic analyses are candid about this: the studies differ too much in measurement methods and protocols, making the results poorly comparable with one another. Both analysis groups pointed to a high risk of bias and were unable to calculate a pooled statistic. As a result, no standardised, clinically validated blood test for immunosenescence yet exists.
Telomere length in immune cells, the protective ends of chromosomes, has also been measured as a potential marker. The pattern makes biological sense: older cells have shorter telomeres. But the variation between studies is so large that telomere length is not yet a reliable measuring stick for clinical practice.
There is one early human study into an agent that may be able to improve the ageing immune system. Low doses of a substance related to the transplant drug rapamycin improved the response to the influenza vaccine in older adults by approximately 20%. The study was conducted by researchers affiliated with the manufacturer. There is too little independent follow-up research to draw firm conclusions from it.
This answer draws on multiple reviews, two systematic analyses and one clinical study. The two systematic analyses are the most directly relevant to the question about blood tests: both encountered such large differences in measurement methods and reporting among the underlying studies that a pooled statistic was not possible. The researchers themselves report a high risk of bias and were therefore unable to identify definitive markers for use in clinical practice. That judgement carries considerable weight: the biological findings on exhausted immune cells, thymus shrinkage and inflammaging are themselves fairly well established, but the translation into a usable blood test has not yet been made.