Researchers have identified more than 100 autoimmune diseases and disorders. Some estimates noted between 80 and 150 distinct conditions. Their prevalence varies. A 2023 population-based cohort study in the United Kingdom involving over 22 million individuals found that about 10 percent of the population is affected by 19 common autoimmune diseases. Moreover, in the United States, data collected by Johns Hopkins University suggest that 3 percent of the U.S. population, or about 10 million Americans suffer from at least one autoimmune disease.
Autoimmune diseases and disorders are not a new phenomenon. They have been documented and recognized for over a century across different scientific publications and medical reports around the world. These include studies on chronic thyroiditis in the early 20th century. However, based on current data, their prevalence and diversity have significantly increased in recent decades. Nevertheless, while it is impossible to pinpoint the exact reason why people develop autoimmune conditions, some suggest an evolutionary cause behind its emergence.
The Cost of Evolution: Theorized Link Between Evolutionary Adaptation to Infectious Pathogens and Modern Autoimmune Disorders
Adaptations Leading to Heightened Immune System
The immune system is designed to protect the body against foreign assailants or pathogens like bacteria and viruses and even defective cells like cancerous cells. However, in certain conditions, it can attack healthy cells, tissues, and organs. This is called autoimmunity, and it is the mechanism behind autoimmune diseases. It involves the production and presence of antibodies directed against normal components of an affected person.
Human populations have long been locked in an evolutionary arms race with pathogens. This is one of the drivers of human evolution. For example, based on several studies, even viral infections that affected pre-human species 2 to 60 million years ago resulted in the integration of their genetic materials into modern human genome. These are called human endogenous retroviruses and they account for at least 8 percent of the human genome.
Evolution may help explain why autoimmune diseases exist. Note that humans developed stronger immune systems to survive deadly infections over thousands of years. Some of these adaptions that boosted resistance also made immune responses more aggressive. Nevertheless, as infectious diseases declined due to improvements in sanitation and advances in medicine, these heightened immune defenses became double-edged swords.
The adaptations in the past against infections created a heightened immune system. A stronger and more responsive immune response was needed to fight deadly pathogens. These adaptations were later passed on to the succeeding generations. It is important to note that those who carried genetic traits that made their immune systems more effective at detecting and destroying these pathogens were more likely to survive and pass on their genes.
However, according to F. Catania and team, evolution should have purged autoimmune diseases since the immune system has been evolving for more than 500 million years. They then proposed that autoimmune response originally evolved in the innate immune system and not in the adaptive immune system. This could support the idea that the modern human immune system is not fully compatible with the modern sanitized world.
Linking These Adaptations to Autoimmune Diseases
A particular research spearheaded by evolutionary geneticist Lluis Quintana-Murci probed about 2900 ancient and modern genomes to understand how European DNA has changed. Results identified 139 spots on the human genome that were influenced by natural selection. Most of these changes occurred due to the response of the immune system to infections. More than 80 percent of positive selection events transpired in the last 4500 years.
The study also underscored the fact that it was around 4500 years ago when exposure to pathogens increased across human communities and epidemics became more frequent and widespread. These were brought by urban growth, expanding agricultural activities and living closer to domesticated animals. The increased exposure to pathogens put evolutionary pressure on immune genes to adapt as fast as possible with less rooms for optimization.
Several variants of the MHC/HLA genes have also undergone positive selection to better detect and respond to certain pathogens. These same gene variants also increase the risk of developing several autoimmune disorders such as ankylosing spondylitis, an inflammatory disease that can result in spinal vertebrae fusing together, and type 1 diabetes, a condition in which the immune system attacks the insulin-producing cells in the pancreas.
Another investigation by J. F. Brinkworth and L. B. Barreiro that used genome-wide association studies underscored the fact that a number of chronic inflammatory and autoimmune diseases are heritable. This means that genetic predisposition is one of their main risk factors. Moreover, based on the referenced studies, the researchers found that chronic inflammation seen in these diseases is largely driven by a small group of immune system genes.
Hence, from an evolutionary perspective, these autoimmune diseases are perplexing because they often reduce reproductive fitness. Natural selection should have eliminated them. However, from a wider perspective, the genes linked to these diseases are also crucial for fighting infections. These genes may have been favored by evolution because they helped humans survive deadly pathogens in the past. This is an example of an evolutionary trade-off.
FURTHER READINGS AND REFERENCES
- Bayersdorf, R., Fruscalzo, A., and Catania, F. 2018. “Linking Autoimmunity to the Origin of the Adaptive Immune System.” Evolution, Medicine, and Public Health. 2018(1): 2-12. DOI: 1093/emph/eoy001
- Brinkworth, J. F., and Barreiro, L. B. 2014. “The Contribution of Natural Selection to Present-Day Susceptibility to Chronic Inflammatory and Autoimmune Disease.” Current Opinion in Immunology 31: 66-78. DOI: 1016/j.coi.2014.09.008
- Conrad, N., Misra, S., Verbakel, J. Y., Verbeke, G., Molenberghs, G., Taylor, P. N., Mason, J., Sattar, N., McMurray, J. J. V., McInnes, I. B., Khunti, K., and Cambridge, G. 2023. “Incidence, Prevalence, and Co-occurrence of Autoimmune Disorders Over Time and by Age, Sex, and Socioeconomic Status: A Population-Based Cohort Study of 22 Million Individuals in the UK.” The Lancet. 401(10391): 1878-1890. DOI: 1016/s0140-6736(23)00457-9
- Department of Pathology, Johns Hopkins University. 2025. “Prevalence of Autoimmune Disorders.” Autoimmune Diseases: Clarity & Facts for Patients. Department of Pathology, Johns Hopkins University. Available online
- Kerner, G., Neehus, A.-L., Philippot, Q., Bohlen, J., Rinchai, D., Kerrouche, N., Puel, A., Zhang, S.-Y., Boisson-Dupuis, S., Abel, L., Casanova, J.-L., Patin, E., Laval, G., and Quintana-Murci, L. 2023. “Genetic Adaptation to Pathogens and Increased Risk of Inflammatory Disorders in Post-Neolithic Europe.” Cell Genomics. 3(2): 100248. DOI: 1016/j.xgen.2022.100248