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Antigen presentation is a vital immune process that is essential for T cell immune response triggering. Because T cells recognize only fragmented antigens displayed on cell surfaces , antigen processing must occur before the antigen fragment can be recognized by a T-cell receptor .
HLA-A projected away from the cell surface and presenting a peptide sequence. The peptide-MHC complex presents a surface that looks like an altered self to the TCR. [11] The surface consisting of two α helices from the MHC and a bound peptide sequence is projected away from the host cell to the T cells, whose TCRs are projected away from the T cells towards the host cells.
Once the exogenous antigen peptide is loaded onto the MHC class I molecule, the complex is exported to the cell surface for antigen cross presentation. There is also evidence that suggest that cross-presentation requires a separate pathway in a proportion of CD8(+) dendritic cells that are able to cross-present.
Antigen presentation stimulates immature T cells to become either mature "cytotoxic" CD8+ cells or mature "helper" CD4+ cells. An antigen-presenting cell (APC) or accessory cell is a cell that displays an antigen bound by major histocompatibility complex (MHC) proteins on its surface; this process is known as antigen presentation.
The latter case induces recognition by antigen-specific Th2 cells or Tfh cells, leading to activation of the B cell through binding of TCR to the MHC-antigen complex. It is followed by synthesis and presentation of CD40L (CD154) on the Th2 cell, which binds to CD40 on the B cell, thus the Th2 cell can co-stimulate the B cell. [11]
Later studies revealed that tissue rejection due to incompatibility is only a facet of the full function of MHC molecules, which is to bind an antigen derived from self-proteins, or from pathogens, and bring the antigen presentation to the cell surface for recognition by the appropriate T-cells. [2]
According to this theory, the chief cell of the immune system is the B cell, activated by recognizing non-self structures. Later research showed that B cell activation is reliant on CD4+ T helper cells and a co-stimulatory signal from an antigen-presenting cell (APC).
After the processed antigen (peptide) is complexed to the MHC molecule, they both migrate together to the cell membrane, where they are exhibited (elaborated) as a complex that can be recognized by the CD 4+ (T helper cell) – a type of white blood cell. [note 7] [20] This is known as antigen presentation.