A group of researchers at the Università della Svizzera italiana gave mice small foot tattoos in black, red or green, then watched under the microscope where the ink went. It travelled fast. On the paper’s own imaging, the pigment reached the lymphatic vessels draining the foot within about ten minutes, and by twenty-four hours it had gathered in the nearest lymph node. The finding appears in the Proceedings of the National Academy of Sciences, published on 25 November 2025, with Santiago F. Gonzalez of the Institute for Research in Biomedicine in Bellinzona as corresponding author. In its own announcement, the institute describes the work as seven years of research involving twelve collaborating groups.
What follows is a reading of the paper, not medical advice, and the work is one study built mainly on a mouse model rather than a settled conclusion about people.
Coverage has run the several findings together into one alarming line: tattoos wreck your immune system. The paper draws finer distinctions than that, and the distinctions are where the interest is.
The mechanism the paper describes
Once the ink reached the node, immune cells called macrophages did what macrophages do: they engulfed the foreign particles. The trouble, according to the authors, is that these cells could not clear the pigment. Electron microscopy showed them packed with ink-filled vacuoles, some bearing structural signs of distress such as membrane damage and blebbing. Cell counts rose in the first hours and then fell. In laboratory dishes, both mouse and human macrophages exposed to the inks underwent apoptotic cell death, with the timing varying by colour.
This disturbance settled slowly. Elevated inflammatory markers turned up in the draining node two months after tattooing, and one alarmin, IL-1 alpha, remained raised in the blood at that same point. An early acute phase in the first days gave way to signals that stayed elevated far longer. A brief flare and a low-grade process that lingers are different things, and both appear here.
The vaccine result, and why it needs careful handling
Most of the attention has landed on what happened when tattooed mice were vaccinated. Given the Pfizer-BioNTech COVID-19 vaccine in the tattooed foot, the animals mounted a weaker antibody response against the spike protein, and the deficit was still present in mice tattooed two months earlier. Human immune cells exposed to ink in a dish showed the same direction of effect.
Read the scope before reading the alarm. In the mouse experiments the vaccine was injected into the tattooed footpad, so it drained through the same ink-loaded node. Think of it as a vaccine passing through tissue already carrying pigment, rather than proof that a tattoo anywhere lowers immunity across the board.
Whether the result extends to other vaccines is something the authors explicitly rule out, because the mechanism differs between vaccine types. They raise, separately, an older concern from the live smallpox vaccination programme, where complications were reported in some tattooed service members. That is context they flag, not something this study tested. Their own position is that the COVID-19 finding needs human validation before it means anything for a clinic.
What the study does not show
Read carefully, none of this shows that tattooed people respond poorly to vaccines. The vaccine work was done in mice, supported by cultured human cells. Human tissue contributes something narrower still: biopsies showing that pigment collects in the nodes in a similar pattern, inside similar ink-laden macrophages and giant cells. That deposition-and-cell-level story holds up. A clinical outcome in living people remains unproven.
Pigment in human lymph nodes is an old observation. It has been documented for decades, and discoloured nodes near tattoos have turned up in imaging often enough to be occasionally mistaken for something worse. What this study adds is a closer look at the cellular machinery: which cells take up the ink, what happens to them, and how long the disturbance lasts in the animal.
On cancer the authors stay careful, while the coverage has run ahead of them. They point to a 2024 Swedish study by Christel Nielsen and colleagues at Lund University, published in eClinicalMedicine, which reported an association between having a tattoo and malignant lymphoma. That study is worth reading with its own limits in view: a case-control design, which is weak ground for causation, and a questionnaire answered by only about half of those approached. Its authors said plainly that more research was needed to establish whether the link is causal. Chronic inflammation near an ink-loaded node is a reasonable thing to ask questions about. A question is still not a finding, and the gap stays open.
Why the regulatory point is the durable one
The more solid takeaway sits away from the vaccine drama. Tattoo inks are strikingly under-characterised for substances placed deliberately inside the body. Black inks generally rely on carbon black; coloured inks often use pigments first formulated for plastics, varnishes or paints. The three used here came from a single major supplier, Intenze, and the authors report checking their composition against the European REACH regulation and screening for impurities such as polycyclic aromatic hydrocarbons and carcinogenic aldehydes.
That REACH check deserves a footnote the paper skips. Since January 2022 the European Chemicals Agency has restricted thousands of hazardous substances in tattoo inks, and from January 2023 it banned two of the most common colour pigments outright, Pigment Blue 15:3 and Pigment Green 7, on the grounds that cancer and other risks could not be ruled out and no safer substitutes were available. Compliance is another matter. A 2024 analysis in Analyst, led by John Swierk’s group at Binghamton University, tested ten green and blue inks sold in Europe as REACH-compliant and found nine fell short, with four still containing banned pigment. A composition check confirms what a manufacturer declares. Certifying that the wider market is clean is a different task.
So this line of work widens a specific gap. Not “tattoos harm your immune system,” but that the field lacks a clear account of what these pigments do after they leave the needle, and that even the labels can mislead. For a practice that, by the study’s figures, involves close to one in five people worldwide and more than thirty per cent in the United States, that gap is worth closing.
What to watch next
Human trials are the obvious next step: whether vaccination near a large, ink-dense area produces any measurable difference in people, and whether the two-month inflammation seen in the animal corresponds to anything meaningful in a human node over years. Until that exists, the sensible reading is the one the researchers offer. The finding is worth taking seriously, and it stops well short of the final word.
If a tattoo raises a specific medical concern, a doctor is the right place to take it, rather than an article about mice.
Carry away the narrowest true version of the vaccine result: a shot injected into an existing tattoo, in a mouse, for one type of vaccine. Everything wider than that is still being worked out.
