17.3 Adaptive immunity  (Page 4/32)

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.