What are T-cells and why do they decline with age?

T-cells are white blood cells that form the core of your defence against viruses, bacteria and cancer cells. There are broadly two types: helper cells that coordinate the immune system, and killer cells that directly destroy infected or malignant cells. Naive T-cells are 'fresh' ones that have never encountered a pathogen before and stand ready to recognise new threats.

Those fresh T-cells are produced in the thymus, a glandular organ behind the breastbone. As you age, the thymus shrinks and gradually loses its function. The production of new naive T-cells therefore drops considerably with age. This is also reflected in blood: studies in humans show that the proportion of fresh T-cells decreases measurably. At the same time, the diversity of T-cells diminishes. This means your body has fewer and fewer different 'faces' available to recognise new invaders, which limits the quality of your immune defence in later life.

Meanwhile, older, exhausted T-cells accumulate. In people around the age of sixty, an average of 64% of killer T-cells are in an aged state. They can barely divide any longer and are more or less out of service. That is more than was previously thought.

On top of that, a specific group of inflammation-promoting T-cells increases. They secrete a protein called granzyme K, which stokes low-grade chronic inflammation in surrounding cells and tissues. This is seen as a hallmark of the smouldering inflammation associated with ageing. Aged T-cells may therefore also contribute to autoimmune diseases such as rheumatoid arthritis, presumably because the internal regulation of the immune system deteriorates.

Whether this tide can be turned remains largely unknown. An early exploratory study observed that BCG vaccination in people with a specific immunological profile slightly reduced the 'biological age' of T-cells, but that is a single observation without a proven causal relationship. Much more research is still needed here.