The field concerning the interactions among cells and molecules of the immune system, and how such interactions contribute to the recognition and elimination of pathogens. Humans (and vertebrates in general) possess a range of nonspecific mechanical and biochemical defenses against routinely encountered bacteria, parasites, viruses, and fungi. The skin, for example, is an effective physical barrier to infection. Basic chemical defenses are also present in blood, saliva, and tears, and on mucous membranes. True protection stems from the host's ability to mount responses targeted to specific organisms, and to retain a form of “memory” that results in a rapid, efficient response to a given organism upon a repeat encounter. This more formal sense of immunity, termed adaptive immunity, depends upon the coordinated activities of cells and molecules of the immune system.
Several types of cells play a role in protecting the body from infection, and they are found primarily in the blood and lymph. Specific immune responses mainly involve the activities of T-lymphocytes and B-lymphocytes, two types of white blood cells. A response is initiated when a pathogen triggers the activity of one or both of the two major types of T-cells: CD4+ cells, also known as helper T-cells (TH); and CD8+ cells, also known as cytotoxic T-lymphocytes (CTL) [see illustration].
When CD8+ T-cells are triggered, they release factors that kill a cell harboring an infectious agent, and they also release cytokines. Virus-infected cells commonly are the targets of cytotoxic T-lymphocytes, since viruses need to get inside a cell in order to reproduce.
The immune system must detect a pathogen before a response can be made. This phase of the response is shown in the interaction between a T-cell and an antigen-presenting cell (APC) [see illustration]. An antigen is a molecule, or portion of a molecule, that is recognized by a T-cell receptor (TCR) or antibody molecule. The cell surface molecules involved in antigen recognition by T-cells are the T-cell receptor and class I or class II molecules of the major histocompatibility complex (MHC). Each T-cell expresses a unique T-cell receptor that will interact specifically with a limited set of antigens. The antigens recognized by T-cell receptors are short peptides bound to MHC molecules. See Histocompatibility
Antigen recognition, as mediated by the T-cell receptor–MHC–peptide interaction, is necessary, but generally it is not sufficient for the initiation of an immune response. Several other molecular interactions occur between molecules on the T-cell surface and those on the antigen-presenting cells. The co-stimulation provided by these additional interactions drives the production of cytokines by T-cells and induces their proliferation.
Once an immune response has been initiated, T-cells and B-cells proliferate and become mature responder cells. These cells do not have the same requirement for co-stimulation once a response is under way. As pathogen elimination nears completion, many of the T- and B-cells involved in the response die. However, a subset of cells remain as memory cells, which can be quickly called into action if the same pathogen is encountered on a future occasion. See Acquired immunological tolerance, Cellular immunology, Immunity, Immunology