The Ancient Weapons Active in Your Immune System Today
18th April 2026
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.