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

Interferons are natural proteins produced by the cells of immune system in response to foreign agents such as viruses, tumor cells and parasites.

It protect non-infected cells from further infection.

Interferons inhibit the viral replication within host cells, activate natural killer cells and macrophages, increases antigen presentation to lymphocytes.

It inducesthe 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.