Some people live decades longer than others, even past 100 years of age. What makes them different?Researchers at the University of Osaka in Japan have found one thing supercentenarians have in common: an abnormally high number of a specific and rare type of cell. CD4 CTLs — white blood cells that are part of the body’s immune response — expand as people reach 100 years of age, according to the research.  In the study, published in the journal Cell Reports, the researchers found that these cells expand and adapt within this older population.  While they typically account for less than 5% of T cells in younger individuals, in supercentenarians — those aged 110 years or older — the median percentage reaches approximately 17.6%. The researchers analysed the blood samples of 28 adults, who had been divided into three age groups: 70–99, 100–109, and 110 and older, and found that the proportion of CD4 CTLs increased with age, with median percentages of 4%, 9.6% and 17.6%, respectively.  Ageing results from the accumulation of molecular and cellular damage over time, leading to a gradual decrease in physical and mental capacity, and a growing risk of disease and death. However, ageing itself is not a disease, nor does it necessarily lead to the failure of essential physiological systems, the authors noted. Supercentenarians are considered a model of healthy ageing: they maintain immune, cardiovascular and epigenomic profiles that appear younger than expected for their age, achieving longevity while avoiding or delaying major age-related diseases such as cardiovascular disorders and cancer. “CD4 CTLs are an atypical and relatively rare T cell population,” said first author Kosuke Hashimoto, an associate professor at the University of Osaka, adding that their marked increase in supercentenarians may provide important clues as to how the immune system is maintained in extreme old age.“The selective expansion of certain T cells suggests that, even in extreme old age, the immune system may continue to adapt to age-related challenges.”  The study suggests these cells may contribute to healthy ageing and longevity by targeting ageing cells that accumulate in the body, and by providing immunosurveillance against tumours. The authors noted that the research, which focused on the presence of T cells circulating in the blood, does not prove that having an abundance of CD4 CTLs prevents cancer or causes a person to live longer. Hashimoto said that the team’s next step is to investigate how these cells behave in human tissue.