According to the latest issue of Nature magazine, the Barcelona Genome Control Center in Spain has used a new technology to track changes in chemical labels on blood stem cell DNA. The results show that from the age of 50, human blood cells will be mainly generated by a small number of stem cell clones, and the immune system will gradually lose its elasticity, making it vulnerable to disease "breaking through the defense line". When young, the human body has 50000 to 200000 active blood stem cells. They produce 100 billion to 200 billion new blood cells every day. As people age, many stem cells gradually "retire", leaving only a few stem cells to continuously clone and replicate new blood cells, and occupy a dominant position. Cloning here refers to a population of cells originating from the same blood stem cell. These cells come from the same ancestral stem cells and therefore carry the same genetic and epigenetic characteristics. And a few dominant clones refer to a few stem cells and their descendants occupying the main part of the blood system. Cloning chemical tags (methylation markers) on cellular DNA helps cells know which genes to turn on or off. When stem cells divide, methylation markers are replicated into their daughter cells, leaving a permanent, natural 'barcode'. Based on this' barcode ', scientists can trace the' family tree 'of stem cells. Scientists have developed a technology called EPI Clone to reconstruct the blood production history of mice and humans, tracking which stem cells contribute to blood production. Research shows that in elderly mice, 70% of their blood comes from a few dominant clones. A similar trend has also been observed in humans: starting from the age of 50, diverse stem cell systems are gradually taken over by a few dominant clones, and this phenomenon becomes more common after the age of 60. That is to say, the blood system used to be like a "hundred flowers blooming", but as one gets older, it becomes "dominated by several families". More importantly, these dominant clones tend to produce myeloid cells, which are immune cells closely associated with chronic inflammation. This may be the root cause of "inflammatory aging" and is also related to the increased risk of diseases such as heart disease, stroke, and leukemia. This discovery helps doctors to observe the aging of patients' blood in advance, thus achieving early detection and prevention. (New Society)