There has been an ongoing debate as to whether viruses are living organisms or simply inert particles of DNA or RNA. A recent study has now established that viruses may have their own immune systems. This of course tends to put them in the category of living organisms. Per the article:
Viruses can acquire fully functional immune systems, according to new research that bolsters the controversial theory that viruses are living creatures...The study, published in the journal Nature, is the first to show that a virus can indeed possess an immune system, not to mention other qualities commonly associated with complex life forms...The use of a complex immune system “doesn’t prove” that viruses are living beings, “but it does add to the argument,” he said.
Reading about this research at Science Daily, I didn't understand what they were saying. When I first learned about viruses almost 5 decades ago, I learned that they had no metabolism. Tobacco mosaic viruses had been stored in crystalline form in a sealed container on a shelf for a decade, with no evident energy flow. When subsequently placed on a living plant, they were infective. I took this to mean that the molecules in a virus don't move around when they're outside of host cells.
Immune systems of animals, on the other hand, are composed of organs, which are composed of tissues, which are composed of cells. True, bacteria can have immunity to phages. Is this intra-cellular function called an immune system? I don't know.
So what does "immune system of a virus" even mean? I can imagine a virus incorporating genetic code, for example the genes for bacterial resistance to phages. But a functioning immune system? How does this work if the molecules inside of the virus aren't even moving around? This does not compute for me.