How do immune cells differentiate between an allergen and infection and thus give respective Ab response?
This question comes down to the type of DC that is presenting the antigen. e.g., if the DC picks up the antigen in an TH2 milieu and travels to a regional LN then it would drive TH2 differentiation. The cytokines of which- e.g., IL-4 and IL-13 can drive B cell isotype switching to IgE. However, in a bacterial infection, these antigens will activate the DC to present to T cells making TH1 cells and the cytokines of which (e.g, IFNy can drive IgG synthesis from B cells and make opsinizing Abs to clearance of the pathogen). Again these DCs would have a TH1 promoting phenotype in order to influence of the differentiation of the T cell that they are presenting too.
Short answer- the phenotype of the DC (depending on the cytokine milieu in gets the antigen in) determines the subsequent differentiation of the T cell and therefore where or not you make IgE (allergy) or IgG (infection control)
