How Do T Cells Discriminate Self from Nonself?
There are two forms of immune responses, innate and adaptive. Individuals from species capable of innate immune responses possess limited repertoires of receptors dedicated to this task. Innate receptors recognizes either stress-induced self-molecules (as exemplified by the NKG2D receptor, see Chapter 2), or bacterial, viral, or protozoan components that are difficult to mutate without an impact on pathogen replicative capacity (as exemplified by the Toll-like receptors, see Chapter 1). The specificity of these receptors is encoded in the germline, and although their expression may be restricted to a certain cell type, they are not clonally distributed (reviewed in Beutler 2003). Most individuals within a species capable of innate immune responses share very similar repertoires of microbial sensors (Pisitkun et al. 2006). Although the ligands of the Toll-like receptors were initially defined as “pathogen-associated molecular patterns,” it should be stressed that in the case of bacteria these ligands are not exclusively derived from pathogens. Therefore, sensors of innate immunity such as Toll-like receptors do not distinguish microbial commensals from pathogens.
KeywordsMajor Histocompatibility Complex Major Histocompatibility Complex Class Major Histocompatibility Complex Molecule Major Histocompatibility Complex Allele Major Histocompatibility Complex Restriction
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