Interferon attacks viruses. Although it cannot save the cell attacked by the virus, it can save the neighboring cells which haven't been invaded by the virus yet. As viruses take over the genetic machinery of the bacterial host, interferon genes are activated to produce interferon molecules. These molecules are then released outside the cell and taken up by neighboring cells. When a virus attacks the neighboring cell, interferon molecules stimulate the cell to turn on genes for antiviral proteins, thereby blocking viral reproduction.
Invertebrates only have macrophages that roam around looking for and getting rid of foreign bodies.
Vertebrates have developed a system which differentiates between self and foreign. The larger the molecular weight, the greater is the antigenicity or immune response. As a rule, small molecules generally don't elicit an immune response, but chemicals such as formaldehyde, poison ivy or oak toxin, penicillin, industrial chemicals and others can bind to larger natural molecules in the cell. The immune system sees these molecules as different and thus, a reaction is elicited. Other examples of antigens are viruses, bacteria, single-celled fungi and protozoans.