Most people who have had a bad cold or dose of the flu will be well aware how feeling physically unwell can also make our cognitive or mental abilities less efficient. This is not simply because we are distracted by aches and pains and feelings of fatigue and general malaise, but partly due to direct relationships between the functioning of our immune system and our cognitive abilities such as memory.
There is good evidence from studies of both animals and people with a range of health conditions of complex interactions between markers of immune system activity and memory function. In general, these studies have found an inverse relationship between signs of immune system activity and measures of memory. So people (and rats) whose blood samples show very high levels of immune cells (e.g. T cells) and biomarkers (e.g. inflammatory cytokines) tend to perform poorly on a test of memory recall. The same biological reaction to bacteria, viruses or cancer cells may, if prolonged, have some deleterious effects on parts of the brain, such as the hippocampus, which are intimately involved in making new memories. Surprisingly though we know little about how these biological and cognitive systems interact in healthy people!
A study reported by researchers at the Auckland University of Technology examined the relationship between measures of immune system activity and memory function in a sample of 30 university students and staff. These people were all participating in a trial of mindfulness but the study reported here took place at baseline before any mindfulness training had commenced.
Participants completed a computerized battery of tests of immediate-, short-, and long-delay memory recall such as learning a list of words. Blood samples were taken from all participants and analyzed for the presence of immune cells including CD4, CD8, and CD69 cells and inflammatory biomarkers including Interleukin-6 (IL-6), C-Reactive Protein, and cortisol. The study found correlations between two biomarkers, CD4 and IL-6, with memory performance. However, CD4 showed a significant negative relationship with short and long-delay memory recall while IL-6 showed a significant positive relationship with long-delay recall.
This study investigated whether associations between immune cell markers and memory recall observed in clinical samples are present in healthy people from the community. The study observed moderately strong relationships between two immune cell markers, but one was a positive and one an inverse relationship. The researchers had predicted only inverse relationships.
This study was based on a small sample of 30 university students and staff at a single point in time. Much further work on a wider range of participants is needed to clarify the complex interrelationships between immune system function and biomarkers. However, the study does suggest that these relationships may work differently in healthy people compared with sick people. In the future, researchers studying the effects of the immune system on human cognition need to study their interaction in both healthy and unwell people to fully understand the phenomena.
Understanding the complex interplay between the immune system and cognition probably needs a more intricate and integrative approach than the dichotomous view of the immune response and its effectors on one side and cognition on the other side. Similar to complex concepts, the human mind usually favors a modular approach which divides a problem into elementary blocks before processing them individually. Although it is a useful approach, there is a definite danger of oversimplification by missing the links connecting those two major blocks: immune system and cognition. This is our next exciting challenge.
These findings are described in the article entitled Associations between immunological function and memory recall in healthy adults, recently published in the journal Brain and Cognition. This work was conducted by Grace Y. Wang, Tamasin Taylor, Alexander Sumich, Fabrice Merien, Robert Borotkanics, Wendy Wrapson, Chris Krägeloh, and Richard J. Siegert from the Auckland University of Technology.