Marie Gilon, an oncology resident physician and Ph.D. candidate at the University of Liège, has spent her research time studying cells your body has probably cursed a thousand times: eosinophils, the white blood cells infamous for making you itch during hay fever season. But a sweeping new review suggests these allergy-fighters may hold an unexpected key to predicting who will survive cancer immunotherapy and who won't.
For decades, eosinophils were pigeonholed as bit players in the immune system—useful only for battling parasites and triggering allergic sneezes. That narrow view is changing. Published in the Journal of Experimental & Clinical Cancer Research, Gilon's synthesis of epidemiological and experimental data reveals something more profound: these cells talk to tumors in ways that can either help or harm cancer's progression, depending entirely on context.
The findings begin at the population level with striking promise. Analyses of large-scale studies suggest that people with higher eosinophil counts face a reduced risk of developing certain cancers in the first place. But once cancer takes hold, the relationship becomes more intricate. Inside a tumor, eosinophils wage a two-part battle against cancer cells. First, they attack directly: these cells carry granules packed with cytotoxic proteins that can kill tumor tissue on contact. Second, they work indirectly by modulating the broader immune response—recruiting and coordinating other immune fighters like lymphocytes to reshape the tumor microenvironment. This dual action explains why the scientific literature sometimes tells contradictory stories about eosinophils, showing them as both cancer suppressors and, in certain conditions, enablers.
The most clinically relevant finding involves immunotherapy. Researchers have noticed a compelling pattern: when cancer patients undergoing treatment with immune checkpoint inhibitors have elevated eosinophil counts—whether measured before or during therapy—they tend to respond better and live longer. This association is strong enough that eosinophil measurement could become a practical tool for identifying which patients are most likely to benefit from immunotherapy in the first place. Doctors could use a simple blood count to flag good candidates and anticipate outcomes before committing patients to treatment.
There is a cost to this immune activation, however. Higher eosinophil counts correlate with a higher incidence of immune-related adverse effects—the side effects that can accompany checkpoint inhibitor therapy. This underscores the delicate balance clinicians must strike: the same cellular activity that predicts better cancer outcomes can also predict complications requiring careful monitoring.
The review reflects a rare collaboration between laboratory science and clinical medicine. Gilon's co-authors—Christine Gennigens, Claire Josse, Vincent Bours, and Guy Jerusalem—bridge the Oncology Department of CHU de Liège and the Human Genetics Unit at GIGA, translating bench findings into bedside applications.
Yet significant obstacles remain before eosinophils become standard biomarkers in cancer care. Eosinophil behavior varies widely across different tumor types, and the field lacks standardized measurement protocols. Without uniform testing methods, it's impossible to establish clear clinical guidelines. Researchers acknowledge that further work is needed to clarify the underlying mechanisms and identify exactly when eosinophils might be reliably used as biomarkers or therapeutic targets.
The evidence paints an emerging picture: eosinophils deserve serious attention at the intersection of immunology and oncology. While the path from research finding to clinical tool remains long, this work hints at a meaningful role for humble allergy-causing cells in the next evolution of cancer medicine.