Interferon is a group of hormone-like peptides produced by cells in response to the presence of viruses, bacteria, parasites, and tumor cells.

Here's a breakdown:

* Viruses: Interferons are a primary defense against viral infections. They activate antiviral mechanisms within cells and prevent the spread of viruses.

* Bacteria: Some interferons can also be produced in response to bacterial infections, contributing to the immune response against bacteria.

* Parasites: Interferons play a role in the defense against parasitic infections as well.

* Tumor cells: Interferons can help control the growth of some tumors.

Essentially, interferons are part of the innate immune system, acting as early warning signals to alert the body of an infection or threat. They trigger a cascade of events that can help prevent the spread of infection and fight off disease.