Stanford researchers find a ‘bomb-like’ immune cell hidden inside flatworms |

What if a single immune cell could destroy dozens of others around it simply by exploding like a tiny bomb? That’s exactly what a team of Stanford researchers has discovered while studying planarian flatworms, tiny creatures famous for their extraordinary ability to regrow lost body parts. Scientists have identified a completely new type of immune cell, named “ruptoblasts,” that defends the body by violently bursting open and releasing toxic substances into its surroundings. Within minutes, these cells can wipe out dozens of nearby cells before disappearing entirely, leaving almost no trace behind. The discovery offers a rare glimpse into an ancient immune strategy that may have existed long before more familiar defences, such as white blood cells, ever evolved.

Stanford researchers discover an immune cell that kills by self-destructing

The discovery began with a simple question: Can flatworms tell the difference between their own tissue and tissue from another worm? Researchers in the lab of Bo Wang, an associate professor of bioengineering at Stanford, set out to answer this by cutting planarian flatworms lengthwise and fusing them with tissue taken from a different worm.The flatworm species used in these experiments, Schmidtea mediterranea, has long fascinated biologists for its ability to regrow an entire body from just a small fragment of tissue. But when Chew Chai, a postdoctoral researcher in Wang’s lab, created these “Frankenstein” worms, she found that the animals rejected tissue from unrelated worms, in a process resembling organ transplant rejection in humans.Instead of relying on immune defences similar to those found in people, the flatworms responded in an entirely different way. While examining the rejected tissue under a microscope, Chai noticed cells that appeared to vanish almost instantly, leaving behind a trail of dead cells in their wake. After ruling out errors in the experiment, the team realised they had stumbled upon a previously unknown immune cell type, which they named “ruptoblasts.“

How ruptoblasts trigger ‘ruptosis’ to destroy nearby cells

According to the study published in Cell, ruptoblasts are triggered by activin, a hormone already known to play an important role in flatworm biology, regulating both regeneration and reproduction.When activin levels spike, often as a result of tissue rejection, infection, or injury, ruptoblasts respond by undergoing a rapid form of cell death, the researchers have named “ruptosis.” The cell’s calcium levels surge sharply, causing it to burst open within seconds to minutes, releasing a cocktail of toxic substances that kill nearby cells before the ruptoblast itself disappears entirely.What makes ruptosis especially unusual is its sheer speed. Other organisms, including some mammalian cells and bacteria, are known to undergo explosive forms of cell death, but these typically unfold over several hours, as cell contents slowly leak out through pores. Ruptosis, by contrast, happens almost instantly, making it, according to the researchers, the fastest form of explosive cell death documented so far.

Why planarian flatworms reveal a never-before-seen immune defence

Ruptoblasts are quite different from the immune cells found in humans and other vertebrates. T cells, natural killer cells, and neutrophils, the cells most associated with fighting infection in mammals, are hematopoietic, meaning they originate in bone marrow. Ruptoblasts, however, are glandular cells that appear to repurpose their secretory machinery for an entirely different, far more destructive purpose.When the team searched for similar cells in other animals, they found ruptoblast-like cells only in basal bilaterians, a group that includes flatworms and diverged from the lineage leading to vertebrates hundreds of millions of years ago, according to the research paper. This suggests ruptosis may represent an ancient immune strategy that vertebrates eventually lost over the course of evolution.One theory is that mammals simply can’t afford this kind of defence. Rupture causes localised tissue damage, and flatworms, with their abundant stem cells and remarkable regenerative powers, can quickly repair that damage. Vertebrates, lacking the same regenerative capacity, may have evolved gentler immune strategies instead.

What ruptoblasts could mean for future medical treatments

To test how powerful ruptoblasts really are, researchers exposed them to E. coli bacteria, human kidney cells, and mouse blood cells. In every case, the ruptoblasts successfully destroyed their targets.Crucially, the damage stayed highly localised. There was no chain reaction spreading to surrounding cells, and no lasting toxicity once the ruptoblast had burst and disappeared. According to senior author Bo Wang, this precision is what makes the discovery particularly exciting from a medical standpoint, since it could potentially be harnessed to design targeted treatments for bacterial infections or even tumours, without harming healthy surrounding tissue, as detailed in Stanford University’s official report on the findings.For now, the discovery mainly highlights how little is known about immune systems beyond a handful of well-studied species. As Wang puts it, countless animals living in environments full of bacteria and viruses likely rely on immune mechanisms scientists haven’t even begun to study. By looking at unconventional organisms like the planarian flatworm, researchers hope to uncover entirely new biological strategies, ones that could eventually inspire fresh approaches to some of medicine’s toughest challenges, from infections to cancer.