Major Histocompatability Complex Definition and Types

Major Histocompatability Complex Definition (MHC) – The group of genes, located on chromosome 6, that determine the human leukocyte antigens (hlas) the body’s cells carry on their cell membranes. HLAs are unique proteins that cell membranes display to identify themselves to the immune system. There are three types of MHC:

  • Class I MHC encodes the HLAs that all nucleated cells and platelets in the body carry to identify them as self cells (the body’s own cells).
  • Class II MHC encodes the HLAs that lymphocytes, macrophages, dendritic cells, and other substances involved in antigen processing carry. These HLAs are fundamental to antibody-mediated immunity and are also responsible for graft vs. host disease and organ rejection in people who undergo organ transplantation.
  • Class III MHC encodes the immunoglobulins from which the immune system forms antibodies.

Major Histocompatability Complex Function

MHC is central to antigen processing. When a macrophage or dendritic cell (phagocytes, also called scavenger cells, in the mononuclear phagocyte system) consumes cellular debris, it displays the antigens of the debris alongside its own HLA. This comparison display allows T-cell lymphocytes to recognize the cellular debris as self or nonself and respond accordingly. Self antigen evokes no reaction; nonself antigen mobilizes the immune response.


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