When is antigen processing an essential prerequisite for an immune response?

Answer 1

Is antigen processing (in your question here) where a cell takes up the antigen and displays it on its membrane (in the MHC molecules) ?

If this is the case, then antigen processing is required for most T-cell immune responses.

Macrophage activation
Hypersensitivity reactions

Answer 2

Antogens(working anti-pathogenics) are needed to respond to pathogens(FOREIGN INVADERS-i.e-viruses) and if you don't have the process working correctly, then the pathogens will prevail and you get no immune response.You would get really sick with no way to fight it off!