Measuring strength of immune responses against a specific pathogen or antigen is appropriate – that is, strong and weak are reasonable attributes to assign to the immune system within such a strictly defined context. However, strong versus weak is a misguided scale for assessing the immune system as a whole.
After all, the immune system functions strongly in allergies, autoimmunities and other types of inflammatory disorders such as IBD. Rather than immune responses being weak in such immunopathologies, they’re wayward (dysregulated) with respect to their choice of targets – they err in deeming otherwise harmless antigens harmful (as in allergies) or err in not tolerating the body’s own antigens (as in autoimmunities) while the immune systems of healthy counterparts consider those very same targets, allergens or self-antigens, benign or at the very least, they deal with them without causing pathology.
Thus the relevant questions are whether the aging immune system
- Remains effective with respect to its capacity to counteract pathogens.
- Remains reliably judicious in its choice of targets.
Aging affects all parts of the body – muscles tend to atrophy, arteries to thicken – while modern diets and lifestyles can engender chronic diseases such as cardiovascular disorders, diabetes, obesity. Stands to reason then that aging would affect the immune system as well, a process called Immunosenescence – Wikipedia.
Additionally, some age well, others not as well. In like fashion, the aging immune system functions differently in different people (1). Those who don’t age well tend to have constant low-grade inflammation or Inflammaging – Wikipedia (2).
Overall, for the following reasons, the immune system’s ability to be effective in its responses and to be judicious in its choice of targets wanes with age (also below from 1, 3, 4).
- Aging goes hand in glove with potentially irreversible maladaptive alterations in metabolism. The resultant cascade effect on nutritional status in turn profoundly affects immune function directly as well as indirectly through its effect on microbiota.
- The body’s immune history, a lifetime of fighting off infections, comes with a price – the memory compartment of the adaptive immune system expands at the expense of its naive counterpart.
- Such shrinking of the naive T and B cell repertoire in turn renders the aging immune system less effective in its anticipatory defense capability – less prepared to fight off newer, unanticipated threats.
- The price the aging immune system pays for its history is compounded by the steep toll persistent viruses such as CMV exact from it on an ongoing basis – the immune system can deal with such persisters handily at younger ages when its regenerative capacity is in full bloom. However, such capacity shrinks dramatically with age.
- As a consequence of creeping ‘immunodeficiency’, such aging-related changes tax the immune system’s ability to be effective and to choose its targets judiciously, making it
- Less capable of fighting off new infections.
- Less stringent in its immunosurveillance against the antigenic products of mutant cells that could be potentially malignant. Such failure could stoke cancer (below from 5).
“the median age for cancer diagnosis in industrialized countries is approaching 70 years of age”
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- Less capable of maintaining full tolerance to its own body, which could result in autoimmunity.
- Epidemiological data suggest such deleterious changes in the aging immune system make the aging body more vulnerable to specific infections. This is why several countries recommend specific vaccines among those >60 years and older.
Anti-influenza immunity offers a handy microcosm for gauging aging-related maladaptive changes in the immune system – it’s least effective among the youngest (in whom the immune system isn’t yet fully developed) and the oldest (below from 5).
Bibliography
1. Boraschi, Diana, et al. “The gracefully aging immune system.” Science translational medicine 5.185 (2013): 185ps8-185ps8. https://pdfs.semanticscholar.org…
2. Franceschi, Claudio, et al. “Inflammaging: a new immune–metabolic viewpoint for age-related diseases.” Nature Reviews Endocrinology (2018): 1. http://www.mr-gut.cn/s/pf/180727…
3. Lee, Naeun, Min Sun Shin, and Insoo Kang. “T-cell biology in aging, with a focus on lung disease.” Journals of Gerontology Series A: Biomedical Sciences and Medical Sciences 67.3 (2012): 254-263. T-Cell Biology in Aging, With a Focus on Lung Disease
4. Fulop, Tamas, Anis Larbi, and Graham Pawelec. “Human T cell aging and the impact of persistent viral infections.” Frontiers in immunology 4 (2013): 271. Human T Cell Aging and the Impact of Persistent Viral Infections
5. Simon, A. Katharina, Georg A. Hollander, and Andrew McMichael. “Evolution of the immune system in humans from infancy to old age.” Proceedings of the Royal Society B: Biological Sciences 282.1821 (2015): 20143085. Evolution of the immune system in humans from infancy to old age