Concept in action
View this animation from Rockefeller University to see how dendritic cells act as sentinels in the body’s immune system.
T cells have many functions. Some respond to APCs of the innate immune system and indirectly induce immune responses by releasing cytokines. Others stimulate B cells to start the humoral response as described previously. Another type of T cell detects APC signals and directly kills the infected cells, while some are involved in suppressing inappropriate immune reactions to harmless or “self” antigens.
There are two main types of T cells: helper T lymphocytes (T H ) and the cytotoxic T lymphocytes (T C ). The T H lymphocytes function indirectly to tell other immune cells about potential pathogens. T H lymphocytes recognize specific antigens presented by the MHC class II complexes of APCs. There are two populations of T H cells: T H 1 and T H 2. T H 1 cells secrete cytokines to enhance the activities of macrophages and other T cells. T H 2 cells stimulate naïve B cells to secrete antibodies. Whether a T H 1 or a T H 2 immune response develops depends on the specific types of cytokines secreted by cells of the innate immune system, which in turn depends on the nature of the invading pathogen.
Cytotoxic T cells (T C ) are the key component of the cell-mediated part of the adaptive immune system and attack and destroy infected cells. T C cells are particularly important in protecting against viral infections; this is because viruses replicate within cells where they are shielded from extracellular contact with circulating antibodies. Once activated, the T C creates a large clone of cells with one specific set of cell-surface receptors, as in the case with proliferation of activated B cells. As with B cells, the clone includes active T C cells and inactive memory T C cells. The resulting active T C cells then identify infected host cells. Because of the time required to generate a population of clonal T and B cells, there is a delay in the adaptive immune response compared to the innate immune response.
T C cells attempt to identify and destroy infected cells before the pathogen can replicate and escape, thereby halting the progression of intracellular infections. T C cells also support NK lymphocytes to destroy early cancers. Cytokines secreted by the T H 1 response that stimulates macrophages also stimulate T C cells and enhance their ability to identify and destroy infected cells and tumors. A summary of how the humoral and cell-mediated immune responses are activated appears in [link] .
B plasma cells and T C cells are collectively called effector cells because they are involved in “effecting” (bringing about) the immune response of killing pathogens and infected host cells.