The Ancient Weapons Active in Your Immune System Today | Quanta Magazine

immunology

The Ancient Weapons Active in Your Immune System Today

Dozens of new discoveries reveal that defenses evolved by bacteria and viruses billions of years ago still define our own innate immune system.

Introduction

By Viviane Callier Contributing Writer

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April 15, 2026

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bacteria biology cells computational biology CRISPR DNA eukaryotes evolution genetic engineering genomics immunology molecular biology proteins viruses All topics

Evolutionary arms races — where one species is pitted against another, driving the evolution of new or more sophisticated weapons as each tries to gain the upper hand — are ubiquitous in nature. One of the oldest and fiercest battles has been waged for billions of years between bacteria and the viruses that infect them. This escalating warfare has selected for bacteriophage viruses (or “phages”) that devise new ways to invade bacterial cells and, in turn, for bacteria that devise new ways to fend phages off. In their attempts to outmaneuver one another, each species will try anything to stay one step ahead.

In recent years researchers have come upon a surprising finding: Some of the machinery that bacteria use to defend against phages exists, almost unchanged, in our own cells. According to dozens of discoveries made over the past decade, the rules of engagement between cells and viruses were written billions of years ago and still largely define how our innate immune system, the first responder to infection, defends us against viruses and bacteria today.

“Seeing that the rules of host-virus interactions are unchanged over billions of years is a really hard thing to digest,” said Philip Kranzusch (opens a new tab), a microbiologist at Harvard Medical School who was one of the first researchers to discover that a key component of human immunity also exists in bacteria.

The pace of evolution for viruses is “insanely high,” he said, compared to that of long-lived, multicellular eukaryotes like ourselves. Indeed, to keep up with the rapid pace of microbial evolution, animals have evolved targeted defenses such as antibodies that adapt to novel viruses as they invade. And yet, strangely enough, our frontline immune defenses seem to share many antiviral tools with bacteria. “Why would the rules be so fixed?” Kranzusch said.

Two recent waves of discovery broke this field open. First, in 2018, researchers reported a variety of novel bacterial defense systems against viruses (opens a new tab), which now number in the hundreds. The second wave, starting (opens a new tab) around (opens a new tab) 2019 (opens a new tab), showed that some of these bacterial mechanisms exist in plant and animal cells, including our own — and that they still work the same way they did in those distant ancestors.

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“Those two things combining together is what really made the field explode: There’s lots of new phage defense systems, and, hey, those systems are directly related and relevant and mechanistically going to tell us information about human immunity,” said Kranzusch, who will receive the National Academy of Sciences Award in Molecular Biology (opens a new tab) for his research in the field at their 163rd annual meeting on April 26, 2026.

These and other (opens a new tab) discoveries (opens a new tab) that (opens a new tab) followed (opens a new tab) reveal an unexplored landscape of human innate immunity — one that could lead to new medical treatments and biotechnological tools, much as the discovery of the bacterial immune system CRISPR-Cas made genome editing possible.

“There’s a lot to do, and it’s really important to do it,” Kranzusch said, “which is why so many labs have jumped in on it.”

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