ISG

Interferon-stimulated genes (ISGs) are the suite of genes activated when a cell detects viral infection and releases interferon signalling proteins. The interferon response is the body's fastest molecular alarm: infected cells broadcast interferon, neighbouring cells upregulate ISGs, and a cascade of antiviral proteins and immune-cell recruiters halts viral spread before the slower adaptive immune system (T-cells, antibodies) can engage. ISGs include well-characterised antiviral effectors such as MX1, OAS1, and IFITM proteins, each of which disrupts viral replication, entry, or translation at the cellular level.

ISG suppression is the mechanism by which many successful respiratory viruses — including SARS-CoV-2, influenza A, and now B3.13 H5N1 — buy time inside the host before the immune system mounts a co-ordinated response. A pathogen that suppresses ISGs is not invisible to the immune system indefinitely; it is delaying the alarm, not disabling it. The window of delay determines how FAR the virus can spread within a host and how infectious the early disease period is.

The May 2026 CDC Emerging Infectious Diseases study on B3.13 (DOI: 10.3201/eid3205.260053) found that the genotype suppresses ISG responses in human nasal epithelium cultures more effectively than historical H5N1 strains. Combined with B3.13's improved nasal replication, this shifts the mechanistic picture: the virus can reach the primary infection site and delay the molecular alarm, giving it more time to replicate before immune clearance begins. ISG suppression does not determine pandemic outcome alone — transmission mode, receptor binding, and population immunity all matter — but it is a key virulence determinant that distinguishes B3.13 from earlier H5N1 strains.