genetic material as templates and instructions to produce more viruses. Once viruses enter the cells, they are largely hidden from circulating antibodies.

To protect the organism against the intracellular phases of viral infection, several populations of lymphocytes, including a subpopulation of inflammatory and killer T cells, collectively isolate and eliminate virus-infected cells. T cells recognize virus-infected cells by means of cell-surface receptors (called T-cell receptors or TCRs) that adhere by molecular complementarity to "flags" on the surface of infected cells. The flags are a class of molecules that make up what is called a major list of compatibility complex (MHC), which picks up degraded fragments (about nine amino acids long) of viral proteins within the cells and bring them to the surface to be detected by TCRs on inflammatory and killer T cells.

Both the magnitude and the quality of the immune responses by these types of T and B cells are regulated by helper T cells. During the course of an infection T and B cells with virus-specific receptors undergo many rounds of cell division; some cell progeny are destined to be immediate effectors of the response, and others are retained as "memory" cells. Long after an infection (or vaccination), the expanded number of memory cells guarantees that a second exposure to the same virus will be met by an expanded response which develops more rapidly, providing effective immunity before serious consequences of the infection develop. During development of the T and B lymphocytes from their precursors, members of the population that have receptors to self are usually eliminated, inactivated, or not expanded. Thus, the adaptive immune system usually ignores self and responds to nonself by providing early and effective immunity and lifelong immune memory. Much is known about this complex system, but much is yet to be learned, so immunology is still an attractive subject for training and research. For example, many immune diseases are known to be the result of mutations or alterations in particular components of the system, whereas others have an unknown etiology.

Immunology attracts diverse life scientists. Perhaps because the cells of the immune system are easily obtained, the system has often been used as a leading-edge subject for studies in other disciplines. Study of homogeneous lymphocyte populations, for example, leads to research in many aspects of signal transduction, wherein cell-surface receptor engagement signals cells to divide, differentiate, or die. It can be argued that we know more about vertebrate developmental immunology than about any other developmental system, including the first isolated stem cell in any system. Much of what is known about cell-surface adhesion and recognition receptors, the genes that encode them, and the evolution of these genes comes from studies of cells of the immune system. That cells can communicate by secreted protein messages called cytokines was elucidated largely through study of cells and cytokines of the immune system.

1.3 Immunology As An Academic Discipline

Immunologists are at work in virtually every life science department or division. But there are very few departments of immunology in academe. The multidisciplinary nature of immunology research is probably a major reason that immunology is so well connected with the more traditional subjects (such as biochemistry, genetics, and microbiology), whose approaches define their disciplines. It also explains the ready translation of discoveries in immunology to such clinical subjects as rheumatology, surgery (in transplantation), endocrinology (in diabetes), neurology (in multiple sclerosis), and allergy.

Although those connections have served immunology and the other subjects well and have probably protected immunologists from the isolation that their jargon could lead them into, immunology might be less of a force in academic politics and less of a presence in the



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