Initiation Of Adaptive T Cell Immunity

Innate immune responses help to control and eliminate infectious organisms, yet they are not always completely effective. However, even if the innate response is not fully protective, it results in the activation of the adaptive immune response. Numerous innate signals (e.g., Toll-like receptor (TLR) signaling, chemokines, and cytokines) attract immune cells to the local tissues where they are activated, causing the more complex, adaptive immune response. To stimulate adaptive immunity, foreign antigens must first be processed into peptide fragments by APC; the resulting fragments associate with MHC class I or II antigens and are presented to the TCR. In most cases, class I MHC binds internally processed foreign antigens, such as cell-processed viral or parasite peptides, whereas MHC class II presents externally generated peptides, such as vaccine peptides (Table 2). CD8+ T cells respond to class I MHC presented foreign antigens; CD4+ T cells respond to class II MHC.[2,3] The way in which the animal's immune system initially reacts to an infectious pathogen is critical; it determines whether a protective, or an ineffective, or even a pathogenic, immune response will be mounted. The intensity of the response to foreign antigen peptides is dependent on the strength of immunostimulation. This is determined by the immunogenicity of the foreign peptide and the strength of the MHC-antigen complex, as well as on the frequency of TCRs that recognize that complex.[2,3]

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