Human bodies contain not only the cells that carry their own genetic material but also cells from other family members, a phenomenon known as microchimerism. These foreign cells, which can originate from a mother, grandmother, twin or older sibling, or even an uncle or aunt, make up approximately 0.1% of all cells in the body. This occurs through a bidirectional transfer of cells between a fetus and its mother during pregnancy, where maternal cells pass into the fetus and vice versa. These transferred cells can remain in the body for decades or even a lifetime. Women who have been pregnant possess cells from each of the fetuses they have carried, even if the pregnancies ended prematurely. These cells can also be passed on to their children. During pregnancy, microchimerism contributes to the establishment of reciprocal immune tolerance, helping the maternal and fetal immune systems learn not to attack each other. Microchimeric cells may also assist in the repair of damaged tissues, similar to stem cells, by responding to signals within the body and migrating to sites of injury. This has been studied in the healing of skin lesions and could potentially play a role in repairing brain damage after a stroke. However, these cells can also contribute to immune system dysregulation and may be involved in the development of autoimmune diseases such as rheumatoid arthritis or multiple sclerosis. The impact of microchimeric cells depends on their quantity and genetic characteristics, which may explain why these diseases more commonly affect women, who often have higher levels of microchimeric cells. Research into microchimerism may lead to new therapeutic approaches, including harnessing the reparative properties of these cells for cellular therapies and understanding how to neutralize their harmful effects on the immune system. For further information, one can explore the work of Nathalie Lambert on the role of microchimerism in autoimmune diseases and Sélim Aractingi on the healing power of fetal cells in mothers. A video titled Microchimerism: foreign cells are in us! by Blob l'extra-média and a book titled The Bushy Cells by science journalist Lise Barnéoud, published in 2023 by Premier parallèle, provide additional insights. Nathalie Lambert is a research director at Inserm and Sélim Aractingi is co-leader of the Skin Biology team at the Cochin Institute.