Chronic stress lowers immune defences in several measurable ways: fewer functional immune cells, a reduced vaccine response and smouldering inflammation. This has been demonstrated in multiple human studies, although large randomised trials are lacking. Short-term stress has precisely the opposite effect and temporarily strengthens the immune system. In practical terms, persistent psychological stress is the harmful variant to address.
The answer depends strongly on how long the stress lasts. Short-term stress, lasting anywhere from a few minutes to a few hours, does not lower your immune defences. On the contrary: the body actually sends more immune cells, such as dendritic cells, neutrophils and lymphocytes, to sites of infection or inflammation. Both the first and second lines of defence of the immune system are briefly strengthened. This is an evolutionarily logical response: in the face of acute danger, the body needs to be ready to recover quickly from injuries or infections.
With prolonged, chronic stress the picture reverses entirely. Multiple human studies show that persistent psychological stress measurably weakens the protective immune response. The balance between the signalling molecules that regulate the immune system, known as cytokines, becomes disrupted. The number of functional protective immune cells declines, while at the same time a smouldering, low-grade inflammation develops in the body that is itself also harmful.
Two concrete consequences of chronic stress for immune defences are well documented. First, the activity of natural killer cells decreases: these are immune cells that directly eliminate viruses and cancer cells. Second, people who are under prolonged psychological pressure produce fewer antibodies after vaccination than people without that stress burden. This means that a flu shot or other vaccine protects them less effectively.
There are also indications that chronic stress increases susceptibility to certain types of cancer, because protective T-cells are suppressed while inhibitory T-cells increase. This relationship has so far been demonstrated mainly at the mechanistic level, meaning in laboratory research into how it could work biologically. Hard clinical evidence that stress directly causes cancer in humans is currently still lacking.
Finally, ageing plays an amplifying role. As people grow older, a persistent mild background inflammation naturally develops in the body, also referred to as 'inflammaging'. Stress contributes to this mechanism via the integrated stress response, and this chronic inflammation is associated with an increased risk of age-related diseases, reduced vitality and a higher likelihood of death. Stress and ageing therefore interact in a harmful way on this point.
All claims are based on two key publications (PMID 24798553 and 20302192) plus one study on inflammaging (PMID 38052484). The strength of evidence for most chronic-stress effects is classified as 'moderate', based on multiple human studies. The cancer evidence is limited and primarily mechanistic. No meta-analyses or large RCTs were used as a source.