Does stress really lower my immune defences?

The answer depends on how long the stress lasts. With short-term stress, lasting from a few minutes to several hours, your immune defences do not drop. On the contrary, your body sends more immune cells to sites of infection or inflammation. Both your first and second lines of defence are briefly strengthened. This makes evolutionary sense: in the face of acute danger, your body needs to be ready to recover quickly.

With prolonged, chronic stress, that picture reverses entirely. Multiple studies in humans show that sustained psychological stress measurably weakens your immune defences¹,². The balance between the signalling molecules that regulate your immune system becomes disrupted. The number of active, protective immune cells decreases. At the same time, a low-grade, smouldering inflammation develops, which is itself also harmful.

Two specific consequences are well documented. First, the activity of immune cells that directly eliminate viruses and cancer cells declines. Second, people with prolonged psychological stress produce fewer antibodies after vaccination than people without that stress burden. A flu vaccine therefore offers them less protection.

There are also indications that chronic stress increases susceptibility to certain types of cancer. Protective immune cells become suppressed, while inhibitory immune cells increase. This link has so far been demonstrated mainly at a mechanistic level, that is, in laboratory research. Hard evidence that stress directly causes cancer in humans is currently lacking.

Finally, ageing plays an amplifying role. As you grow older, a persistent background inflammation naturally develops in your body. Stress contributes to this mechanism through what is known as the integrated stress response³. This chronic inflammation is associated with a higher risk of age-related diseases and a higher probability of death. Stress and ageing therefore interact in a detrimental way on this point.