When a cell is infected by a virus, is produced and is secreted from the infected cell to protect nearby uninfected cells by binding to surface receptors and initiating synthesis of antiviral proteins.

The given blank can be filled with interferons.

Explanation:

The signaling molecules or the proteins that get discharged by the cell infected by the virus to protect the adjacent cells by stimulating a safeguarding defense mechanism are termed as interferons.  

The mentioned proteins obstruct the proliferation of viruses and thus helps the cell in getting rid of the infection. These molecules also assist in stimulating the macrophages and the natural killer cells.  

age range would be the answer

a physical change is being made.