A balanced immune response requires precise regulation
One of the primary purposes of the immune system is to recognize and respond to molecular patterns expressed by pathogens. Innate and adaptive immunity must be highly regulated, as responses of increased intensity or prolonged duration may lead to collateral tissue damage and self-reactive responses that initiate autoimmune disease. Many mechanisms have evolved to regulate both the initiation and termination of innate and adaptive immune responses to maintain efficacious removal of the infection followed by resolution of tissue inflammation that avoids responses to self-antigens.
Several factors contribute to the development of a successful immune response to infection. First, the development of the innate and lymphoid cells needed for effective responses to intracellular versus extracellular infection are orchestrated by the inflammatory tissue signals generated by infectious agents within the tissue environment (1). Protective T cell responses are initiated by DCs that emigrate from the infection site to secondary lymphoid organs. These DCs not only present peptide/MHC complexes to trigger naive T cell activation, but also produce soluble factors programmed by environmental cues delivered to the DC at the inflammatory tissue site. Once the infection is eliminated, T cell responses are attenuated by regulatory mechanisms that include apoptosis of the effector cells due to the absence of antigen and the activities of CD4+ Tregs. Macrophages also traffic to sites of infection and tissue inflammation, where these cells undergo functional differentiation dependent on environmental cues, as well as participate in clearance of the infection and subsequent resolution of the inflammation and tissue injury. Collectively, these processes ensure that the effector functions expressed will be those that can most efficiently destroy the cause of the infection and then resolve the inflammation to restore tissue integrity.
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A second and distinct level of regulation occurs within the cell upon receptor engagement of ligands and the delivery of cytokine signals, which together promote the development of the innate and lymphoid cells that will most efficiently deal with the infection. Multiple steps occur between the triggering of receptors and the induction of gene expression, and they include the activation of several kinases and/or phosphatases. Moreover, each of these enzymes has inherent chemical regulatory mechanisms to avoid aberrant triggering of effector functions within tissues. Activation of these intracellular enzymes is often regulated by cations, sequestered within cytoplasmic stores and/or transported into cells through membrane channels, and the availability of these cations is itself regulated.
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