chemistry1111
Hello!
Unfortunately, there isn't a be-all-end-all method for determining whether a pathogen is intracellular or extracellular, but this isn't usually important, and if it is it will probably be given information [in a question]. However, if needed, you can always use logic. For example, helminths can't be intracellular as they are simply too massive.
All pathogens (with the exception of prions) are subject to Adaptive Humoral Immunity responses, but only intracellular pathogens will be subject to (Adaptive) Cell-Mediated Immunity responses. This makes sense, as all pathogens will have to be outside of a host cell eventually -- even viruses need to leave their host cells to infect new ones --, but only those that infect cells can be eliminated by apoptosis of their host cells.
Helper T cells are activated when their receptors bind to ligands (MHC class II) on activated Antigen Presenting Cells (e.g. Dendritic Cell). Cytokines, such as Interleukins, are involved, but this cannot occur independent of APC binding -- in other words the binding of the Naive Helper T cell and APC is the first and crucial step.
