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From Naive to Memory T Cells

Specific immune protection against pathogens is achieved by the development of a diverse set of lymphoid cells whose products can target the organisms for destruction and clearance: A spectrum of highly specific antibodies (Ab)# produced by the progeny of B lymphocytes and T cells with receptors for antigenic determinants of the pathogen, seen as peptides complexed with major histocompatibility complex (MHC) molecules. The Ab can bind to the pathogen while it is pre.sent extracellularty, while the T cells can detect infected cells. When a pathogen infects an animal that has not previously encountered a similar organism, a complex, interdependent series of cellular responses occurs which can generate an effective specific immune response only after a period of 4-7 days. During this time the protection against the infectious organism is primarily limited to the largely nonspecific components of innate immunity and the consequence for the host can be the particular illness associated with the pathogen. The disease is a combination of the direct pathological effects of the infection, coupled with effects of the innate immune response, which can also be injurious. These innate protective mechanisms, which include natural killer cells and induction of granulocytes and macrophage products, are often only partially effective allowing the pathogen to continue replicating and/or infecting cells. The specific effecter response that develops must be sufficiently robust to deal with a large load of infectious organisms. Achieving destruction of the pathogen may require not only initial development of T and B effector populations, but also their further expansion to a size that is sufficient to generate enough Ab and