Two Types of Helper T Cells
When helper T cells are activated during the course of an immune response, they can develop into one of two types of cells: type 1 helper cells, and type 2 helper cells.
The factors that determine what type of helper cell is activated are not fully understood. However, it has long been known that each type of cell is associated with different arms of the immune response, and that each helper cell type stimulates the response which is most effective at eliminating the pathogen involved.
Each helper cell type stimulates the required response by secreting a particular pattern of cytokines, to stimulate and enhance the activity of immune cells which can effectively fight the infection.
Type 1 helper cells secrete cytokines such as interferon-gamma, which improves the killing efficiency of macrophages, and improves the proliferation efficiency of cytotoxic T cells. This type 1 immune response is produced as a result of infection with viruses and other intracellular pathogens.
Type 2 helper cells secrete a wide range of interleukins, which stimulate proliferation of B cells, and stimulate antibody production. This type 2 response is produced in response to extracellular bacteria and toxins.
In both cases, the activation of helper T cells, and therefore the activation of the adaptive immune response, stimulates the development of immunological memory, which provides protection for the body the next time the same pathogen is encountered.