MHC

Examples of MHC in the following topics:

• Natural Killer Cells

  • Natural killer cells are part of the innate immune response that recognize abnormal MHC I molecules on infected/tumor cells and kill them.
  • If the cell is infected, the MHC I molecules display fragments of proteins from the infectious agents to T-cells.
  • An infected cell (or a tumor cell) is often incapable of synthesizing and displaying MHC I molecules appropriately.
  • The reduced MHC I on host cells varies from virus to virus and results from active inhibitors being produced by the viruses.
  • This process can deplete host MHC I molecules on the cell surface, which prevents T-cells from recognizing them, but which NK cells detect as "unhealthy" or "abnormal" while searching for cellular MHC I molecules.

• Cell-Mediated Immunity

  • T cells become activated towards a certain antigen once they encounter it displayed on an MHC II.
  • To do so, a T cell will become activated by interacting with an antigen of the infecting cell or virus presented on the MHC II of an APC.
  • To recognize which cells to pursue, TC recognize antigens presented on MHC I complexes, which are present on all nucleated cells.
  • MHC I complexes display a current readout of intracellular proteins inside a cell and will present pathogen antigens if the pathogen is present in the cell.
  • Lymphocytes of the adaptive immune response must interact with antigen-embedded MHC class II molecules to mature into functional immune cells.

• Antigen-presenting Cells: B and T cells

  • Instead, they recognize antigens presented on major histocompatibility complexes (MHCs) that cells use to display which proteins are inside of them.
  • If a cell is infected, it will present antigenic portions of the infecting pathogen on its MHC for recognition by T cells, which will then mount an appropriate immune response.
  • Unlike antibodies, which can typically bind one and only one antigen, T cell receptors have more flexibility in their capacity to recognize antigens presented by MHCs.
  • A T cell receptor spans the membrane and projects variable binding regions into the extracellular space to bind processed antigens via MHC molecules on APCs.

• Immunological Memory

  • After initially binding an antigen to the B cell receptor (BCR), a B cell internalizes the antigen and presents it on MHC II.
  • A helper T cell recognizes the MHC II–antigen complex and activates the B cell.

• Humoral Immune Response

  • When a B cell encounters the antigen that binds to its receptor, the antigen molecule is brought into the cell by endocytosis, reappearing on the surface of the cell bound to an MHC class II molecule.
  • This activation of the helper T cell occurs when a dendritic cell presents an antigen on its MHC II molecule, allowing the T cell to recognize it and mature.
  • The helper T cell binds to the antigen-MHC class II complex and is induced to release cytokines that induce the B cell to divide rapidly, making thousands of identical (clonal) cells.

• Immunodeficiency

  • Dozens of genetic disorders result in immunodeficiencies, including Severe Combined Immunodeficiency (SCID), bare lymphocyte syndrome, and MHC II deficiencies.

• Regulating Immune Tolerance

  • The pocket contains antigen-presenting cells, such as dendritic cells, which engulf the antigens, then present them with MHC II molecules on the cell surface.