What are interferons? How do interferons check infection of new cells?

(N/A) Interferons are natural proteins produced by the cells of the immune system in response to foreign agents such as viruses,tumor cells,and parasites.
They protect non-infected cells from further infection.
Interferons inhibit viral replication within host cells,activate natural killer cells and macrophages,and increase antigen presentation to lymphocytes.
They induce the resistance of host cells to viral infection.
When the antigen is presented to matching $T$-cells and $B$-cells,these cells multiply and remove the foreign substance.