Innate immune system cytotoxicity cells

A. Overview The innate immune system initiates the defense against pathogens and antigenic insult. It involves the concerted actions of complement components, lysozyme, macrophages, and neutrophils. If the innate response is inadequate, the adaptive immune response is mobilized. This culminates in the activation of T lymphocytes, the effectors of cell-mediated immunity and the production of antibodies, by activated B lymphocytes, the effectors of humoral immunity. The cell types involved in immune responses can be identified by specific cell surface components or clusters of differentiation (CDs). For example, T helper cells bear the CD4 protein complex, whereas cytotoxic T lymphocytes express the CDS protein complex. Clusters of differentiation also can be used to characterize other types of hematopoietic cells, including precursors of granulocytes, megakaryocytes, and erythrocytes (Chapter 33). 

NK cells are cytotoxic lymphocytes that play a role in the innate immune system, not requiring antigenic specificity and NK T cells are lymphocytes that bridge the adaptive and innate immune systems, recognizing antigen presented by CDid. 

In the early stage of viral infection there is an intensive race between the virus and the host s defense systems, which are mediated through (1) nonspecific or innate immune defenses by interferon, natural killer (NK) cells, complement cascade, apoptosis, and macrophages (2) specific or adaptive immune defenses by cytotoxic T lymphocytes (CTL), helper T lymphocytes, and anti-viral antibodies (Ploegh 1998). The conclusion of this race will depend on how rapidly the virus can infect and/or replicate before it is cleared by an active immune system. 

Sinee the initial finding that IFN-y secretion from activated T cells can induce indirect cytotoxicity by release of reactive species (later identified as NO and per-oxynitrite) from macrophages (Nathan et al. 1983 Nathan et al. 1984) an increasing amount of data on the role of NO in the immune and inflammatory systems have accumulated. NO has been shown to influence different immune functions both during innate and adaptive immune responses, including T-ceU activation and proliferation, cytokine production, APC expansion and maturation, central and peripheral tolerance, Th cell differentiation, as well as T-ceU apoptosis (Brito et al. 1999 MacMicking et al. 1997). 

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