TH-1 cells are mostly involved in cellular immunity; as TH-2 cells are mainly involved in humoral immunity. If a patient suffers from certain kinds of leprosy, the cytokine that you would administer to the patient would be gamma-interferon. A naive T-cell becomes a TH-1 cell and it make gamma-interferon. The gamma interferon first activates macrophages to make them better killer cells. In addition to the cellular immune response, IgG may be produced to accelerate the elimination of the bacteria and increase phagocytosis. TH-1 cells are also known to recruit cells from the bone marrow.
TH-2 cells make Interleukin 4 and Interleukin 13 which drive B cells to produce IgE (which binds to Mast cells) to fight parasitic infections such as worms). TH-2 cells also migrate to the sit of infection to degranulate the cells. It also stimulates Interleukin 5. Interleukin 5 increases the growth and differentiation of IgE cells; whose synthesis additionally promotes mucosal immunity in parasympathic infections.
The cytokines that are secreted to the location where the T-lymphocyte is located is what determines if a cell becomes a TH-1 cell or a TH-2 cell. If Interleukin-12 is predominate (produced by dendritic cells or macrophages) where the T-lymphocyte is present, the cell goes into the pathway to become a TH-1 cell. But, if Interleukin 4 (made by Natural Killer Cells) is present where the T-lymphocyte matures, the cell goes down the route to become a TH-2 cell.