Helper T lymphocytes are specialized white blood cells that are vital to the proper functioning of the immune system. They respond to pathogens and diseased cells by identifying the problem and alerting more active cells to address it. After a helper T cell has identified the threat, it differentiates into one of four specific kinds of helper T cells to better assist the immune system. Th1, Th2, Th17 and Tfh cells are all specialized helper T lymphocytes that target and assist different immune cells depending on what is needed to defend the body.
Lymphocytes are white blood cells whose function is to fight disease and infection. The main types of lymphocytes are distinguished based on their function and where they mature. The body's B cells mature in bone marrow and, when activated, they make antibodies. T cells mature in the thymus gland near the heart. They specialize into cytotoxic cells, or "natural killer" cells, and helper T lymphocytes.
Helper cells cannot directly attack pathogens, but their role is vital to the immune system. This is because they are able to adapt to the specific characteristics of the pathogen to which they are responding. This allows the helper T lymphocytes to activate only those additional immune system features that are needed to respond effectively.
Th1 helper cells protect the body from pathogens that have already infected cells, such as viral infections, by activating macrophages that swallow the cell and cytotoxic lymphocytes that poison it. Only macrophages and cytotoxic cells that are specific to the invading pathogen are activated. This ensures that healthy cells remain unharmed. After being activated by the Th1 cell, the macrophage is cued to destroy what it has swallowed and to seek out additional similar pathogens. If it cannot, then the activated cytotoxic cells inject an enzyme into the diseased cell to kill it.
For pathogens, such as bacteria, that reside outside of the cell, different types of helper T lymphocytes are activated. Th17 cells provide the first line of defense by causing threatened cell boarders to swell in an attempt to prevent the pathogens from entering. Th2 helper cells then trigger B cells to produce specific antibodies to fight the invader, and Tfh cells stimulate other B cells to create plasma that circulates the appropriate antibodies to the surrounding cells and reduces the spread of the infection.
Diseases that attack T helper cells illustrate their vital role in the immune system. The human immunodeficiency virus (HIV) takes over helper T lymphocytes, uses them for reproduction and renders them useless for fighting diseases. This disables the immune system because the helper cells are not available to activate it. Conversely, autoimmune disorders that are linked to inflammation, such as multiple sclerosis and arthritis, appear to be linked to excessive Th17 lymphocyte activity. With these disorders, the Th17 cells cause too much inflammation for the cells to function properly.