Upon activation, naïve Th cells become Th0, since they have both Th1 and Th2 characteristics. With further stimulation Th0 cells deviate either towards Th1 or Th2. Th1 and Th2 cells are classified based
on the pattern of cytokines that they secrete (see table below). If the Th cell secretes mainly IL2 and INF-g, it is termed a Th1 cell. If the T cell secretes mainly IL4, IL10, and IL13 it is termed a Th2 cell.

   NOTE:     -    ThP is a primary activated naive Th cell. It secretes IL-2 only.
                         IL-2 is required for T cell mitosis.
                    -     IL-2 is unique to Th0 and Th1.
                    -     IL-4 is unique to Th0 and Th2.  

                -    For a list of Cytokines function  

• The initial critical stimulators that favor Th1 vs. Th2 responses are largely unknown. Some proposed determining factors are: 
  A.    Nature of the antigen: Intracellular viral antigens favor Th1 and extracellular bacterial antigens favor Th2.
  B.    Concentration of antigen: Low conc. favors Th1 and high conc. favors Th2.
  C.    Presence of NK and Macrophages derived cytokines.
          Macrophages can produce IL-12 (NK activator) or TGF-beta (Th inhibitor).
          NK cells can produce INF-gamma (Th2 inhibitor).

• Different cytokines activate different arms of the immune system. Th1 cytokines activate macrophages, NK cells, and cell mediate immunity (CMI), plus the secretion of certain Ig isotypes. Th2 cytokines tend to favor isotype switching in the humoral immune response. In addition, Th2 cytokines depress macrophage activation and CMI.