Scientists have identified a previously unknown immune defense mechanism that enables cells to destroy invading viruses and bacteria on their own. The discovery could reshape our understanding of how the body fights infections and may eventually pave the way for new therapeutic approaches.

The findings were published in the journal Molecular Cell. The international research team has named the newly discovered mechanism antibody-directed xenophagy (ADX).

Traditionally, the immune system operates outside cells. Antibodies attach to viruses or bacteria in the bloodstream and help immune cells recognize and eliminate threats. However, some pathogens manage to evade these defenses and enter healthy cells.

The researchers found that the battle does not end there.

Inside cells, a specialized protein called TRIM21 acts as an additional line of defense. It can detect antibodies that are already attached to an invading virus or bacterium. Once it receives this signal, TRIM21 effectively raises the alarm and tags the intruder with a molecular marker known as ubiquitin. This triggers a cellular self-digestion process in which the cell destroys the harmful invader from within.

Scientists compare the mechanism to an internal security system that becomes active when a pathogen succeeds in breaching the body's outer defenses.

Experiments showed that the newly identified pathway can target both adenoviruses and bacteria from the Salmonella genus. Importantly, the mechanism functioned in the same way against these very different types of pathogens, suggesting it may represent a broad-spectrum defense strategy.

To assess the significance of the discovery, researchers examined multiple human cell lines and conducted experiments in mice. The results revealed that TRIM21 is present in virtually all tissues of the body and is produced at elevated levels during infections. This suggests that the protective mechanism operates throughout the organism rather than being limited to specific organs.

According to Leo James, who led the study at the MRC Laboratory of Molecular Biology, the absence of TRIM21 significantly weakens the body's natural defense against viruses. This indicates that the newly identified pathway is not merely a backup system but a fundamental component of immunity.

The researchers believe the discovery could have important medical implications. In the future, it may be possible to develop therapies that deliberately coat dangerous viruses and bacteria with antibodies, allowing TRIM21 to recognize threats more rapidly and trigger their destruction inside infected cells.

The findings provide new insight into how the immune system continues to protect the body even after pathogens have successfully entered cells, revealing an additional layer of defense that had previously gone unnoticed.