1. When our body receives a pathogenic antigen from a vaccine, is it ingested by a phagocyte and then presented to Th and B cells, or are these cells just activated by recognizing the antigen, without APCs?
Th cells are activated by an pathogen being engulfed and its antigens presented on a MHC 2 molecule by an Antigen Presenting Cell (APC) ie. a macrophage or dendritic cell.
B cells are activated by free antigens - those just floating throughout the body, not presented on MHC2
Th cells cannot recognise antigens that are not presented on MHC2 and B cells cannot recognise antigens presented on MHC2.
2.With an allergic immune response, how are B cells activated? Do they require APCs or Th cells at all?
Initially it is like a normal immune response. This is called sensitisation. As a result of this IgE antibodies against the allergen are produced. These IgE antibodies bind to mast cells, when the allergen enters the body again it binds to these antibodies, causing the mast cells to degranulate (release histamine). This is the allergic response.