Last week Science has an update on differential antigen processing by DCs including a key sentence on immature DCs:

Cultured immature DCs capture antigen but only process and present it on MHC II after exposure to inflammatory stimuli or TLR ligation.

Although the authors were not aware of current allergy research, they perfectly summarize how vitamin D renders DC immature, while hygiene (infections or LPS farm exposure ) may antagonize it.