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antigen presentation
Antigen presentation describes a vital process of the immune system. Immune cells cannot "see inside" other cells, which may be infected with viruses or bacteria, and thus rely on information conveyed by fragments of intracellular components being presented on major histocompatibility complex (MHC) molecules on the cell surface. Likewise, many pathogens are too large to be recognised directly by immune cells, and must first be digested into smaller fragments that can be presented by specialised antigen-presenting cells (APCs).
==Intracellular antigens: Class I==
Because disease processes can develop inside the cells of the body and this is hard to recognise from the outside, cytotoxic T cells run a regular molecular health screening program for nucleated cells in the body to ensure they are free from intracellular diseases, thus fit for service. Most often viruses can replicate within host cells; alternatively intracellular bacteria can survive or oncogenes can be translated into tumourigenic gene products. To facilitate screening out these kinds of intracellular health problems, the host cell voluntarily presents bits of its cytosolic content on the outside of the cell for cytotoxic T cells to inspect during periodic visits. To do this, the host cell digests its own cytoplasmic proteins by a specialized enzyme complex, the proteasome, into small peptides. A specialized carrier, the transporter associated with antigen processing (TAP) complex moves the peptide into the endoplasmic reticulum, allowing the antigenic peptide to be coupled to an MHC Class I molecule and transported to the cell surface. in the endoplasmic reticulum, the peptide is anchored to the peptide-binding groove on the floor of the α1-α2 heterodimer of the MHC class I molecule.
CD8+ Cytotoxic T cells are programmed to recognise peptides coupled to the MHC Class I molecules on all nucleated cells. Cytotoxic T cells (also known as TC, killer T cell, or cytotoxic T-lymphocyte (CTL)) are a population of T cells that are specialized for inducing the death of other cells. Recognition of antigenic peptides through Class I by CTLs leads to the killing of the target cell, which may be infected by virus, intracytoplasmic bacterium, or are otherwise damaged or dysfunctional. On the other hand, tolerated peptides from physiological protein turnover in healthy cells will be ignored. By inducing cytotoxicity in the diseased cells, cytotoxic T cells ensure the body remains healthy; the eliminated cells can be replaced by healthy cells.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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