A viral reel by Dr. Harini Bhatt (Pharm D) on her Instagram account @tilscience, has drawn global attention to the case of Ana Paula Martins, a Brazilian woman whose blood carries XY chromosomes despite her body being genetically XX. The extraordinary finding marks the first reported case of spontaneous chimerism in humans.
Ana Paula Martins’ journey into medical history began in 2022, after she experienced a miscarriage. During routine follow-up, her gynecologist ordered a standard karyotype test, which analyzes chromosomes from blood samples. To everyone’s surprise, the results showed XY chromosomes—genetically male. Believing it to be a lab error, doctors repeated the test, only to receive the same result.
What made the finding even more astonishing was that Martins is biologically female in every respect. She has a uterus, functioning ovaries, and had always presented as female. The mystery deepened: how could someone who is clearly female have blood cells that carry male DNA?
Chimerism is a rare condition in which a person carries two or more genetically distinct cell populations. It may arise spontaneously—often through twin-to-twin cell transfer in the womb—or after medical procedures like bone marrow transplants. 1
Scientific reports highlight its varied presentations. A study titled Spontaneous 47,XX,+21/46,XY Chimerism: A Case Report (BMC Medical Genetics, 2012) described a fetus carrying both XX and XY cell lines, while Case of a Phenotypically Normal Woman with 46,XX/46,XY Chimerism (PubMed, 2024) detailed a 30-year-old woman whose blood contained XY cells despite her germ cells being XX.
These cases show that chimerism can be silent or visibly expressed, depending on where the genetic differences occur. Ana Paula Martins’ case is extraordinary because her body is entirely female (XX), while her blood alone carries her twin brother’s XY DNA—the first reported example of spontaneous chimerism of this kind. 2
The mystery began to unravel when Martins told researchers that she has a twin brother. Genetic testing revealed something extraordinary. As Dr. Harini Bhatt explained in her reel: “Martins’ DNA in her skin, mouth, eyes, etc. is 100% hers. But in her blood, she is her brother.”
Only Martins’ blood carried the genetic markers identical to her twin. Doctors believe this happened in the womb, when the twins’ placentas connected and their blood vessels intertwined. This led to what scientists call fetal-fetal transfusion, where her brother’s blood cells became part of her circulatory system.
Even more remarkable, Martins’ immune system has tolerated these foreign XY cells throughout her life without rejecting them. As the reel noted, “What’s medically incredible is that Ana Paula’s immune system has tolerated her brother’s XY blood cells for her entire life without attacking them as foreign tissue.”
Ana Paula Martins’ ability to tolerate her twin brother’s XY blood cells for her entire life highlights a remarkable aspect of human immunity: the capacity to coexist with genetically distinct cells. Research in transplantation medicine provides parallels for understanding this phenomenon. A study published in Frontiers in Immunology discusses how mixed chimerism—where an individual carries two genetically distinct cell populations—can induce immune tolerance, allowing organ transplant recipients to accept donor tissues without lifelong immunosuppressive therapy. 3
Similarly, research on blood group chimeric twins, published in Transplantation Proceedings, demonstrates that chimerism can modulate the immune system, promoting tolerance even between genetically non-identical individuals. Martins’ case appears to be a natural example of such tolerance, offering unique insights for immunology and transplantation research. 4
Beyond the genetics and medical marvel, Ana Paula Martins’ story is one of resilience and wonder. Despite carrying her twin brother’s DNA in her blood, she has lived a full, healthy life, even becoming a mother.
As Dr. Harini Bhatt noted in her viral reel, “The human body is truly amazing.” Her case not only challenges what we know about biology but also celebrates the adaptability of life itself.
For scientists, it opens new avenues of research into immune tolerance and chimerism; for the rest of us, it is a reminder that the human body can surprise us in extraordinary ways. Martins’ journey, shared widely through the reel, bridges the gap between cutting-edge science and human experience, making this rare phenomenon both relatable and inspiring.
References:
Chen, C. P., Y. T. Chen, W. Wang, and W. H. Wang. “Spontaneous 47,XX,+21/46,XY Chimerism: A Case Report.” BMC Medical Genetics 13, no. 85 (2012). https://bmcmedgenet.biomedcentral.com/articles/10.1186/1471-2350-13-85.
Massaad, I., L. Moussa, J. Elia, R. Wakim, and N. Khoury. “A Phenotypically Normal Woman with 46,XX/46,XY Chimerism: A Case Report.” Case Reports in Genetics, 2024. https://pubmed.ncbi.nlm.nih.gov/38420683/.
Passeleu, Laura, Judith D. Norton, and Kenneth L. Brayman. 2020. "Tregs and Mixed Chimerism as Approaches for Tolerance Induction in Islet Transplantation." Frontiers in Immunology 11 (December). https://doi.org/10.3389/fimmu.2020.612737.
Szymanski, I. O., C. A. Tilley, M. C. Crookston, T. J. Greenwalt, S. Moore, and J. R. Bove. 1975. "Blood Chimeric Twins: An Example of Blood Cell Chimerism." Transplantation Proceedings 7 (1): 235–38. https://pubmed.ncbi.nlm.nih.gov/1095943/.
(Rh/ARC/MSM)