The major histocompatibility complex (MHC) is a collection of genes coding for MHC molecules found on the surface of all nucleated cells of the body.
MHC molecules display a molecular fraction called an epitope and mediate interactions of leukocytes with other leukocytes or body cells. The MHC gene family provides an extensive amount of genetic diversity because:
- The MHC’s genetic encoding is polygenic,
- MHC genes are highly polymorphic and have many variants,
- Several MHC genes are expressed from both inherited alleles (variants).
In humans, the MHC region occurs on chromosome 6. Human MHC class I and II are also called human leukocyte antigen (HLA).
HLA MHC complex: The human leukocyte antigen (HLA) system is the name of the major histocompatibility complex (MHC) in humans. The super locus contains a large number of genes related to immune system function in humans. This group of genes resides on chromosome 6, encodes cell-surface antigen-presenting proteins and has many other functions.
There are two classes of MHC molecules involved in adaptive immunity, MHC I and MHC II. MHC I molecules are found on all nucleated cells; they present normal self-antigens as well as abnormal or nonself pathogens to the effector T cells involved in cellular immunity. In contrast, MHC II molecules are only found on macrophages, dendritic cells, and B cells; they present abnormal or nonself pathogen antigens for the initial activation of T cells.
Key Points
• Diversity of antigen presentation, mediated by MHC classes I and II, is attained in three ways: (1) the MHC’s genetic encoding is polygenic, (2) MHC genes are highly polymorphic and have many variants, (3) several MHC genes are expressed from both inherited alleles.
• Human MHC class I and II are also called human leukocyte antigen (HLA).
• The MHC genes are highly polymorphic; this means that there are many different alleles in the different individuals inside a population.
Key Terms
polygenic: Having an infinite number of derivatives at a point (otherwise it is monogenic)
Major histocompatibility complex (MHC): is a collection of genes coding for glycoprotein molecules expressed on the surface of all nucleated cells.
MHC I: molecules are expressed on all nucleated cells and are essential for the presentation of normal “self” antigens. Cells that become infected by intracellular pathogens can present foreign antigens on MHC I as well, marking the infected cell for destruction.
MHC II: molecules are expressed only on the surface of antigen-presenting cells (macrophages, dendritic cells, and B cells). Antigen presentation with MHC II is essential for the activation of T cells
polygenic: phenotype is influenced by more than one gene
polymorphic: genetic variation resulting in the occurrence of several different forms or types of individuals among the members of a single species
allele: different versions of the same gene
epitope: the part of an antigen molecule to which an antibody attaches itself
leukocyte: white blood cells
adaptive immunity: a specific type of immunity developed over time
B cell: a lymphocyte, developed in the bursa of birds and the bone marrow of other animals, that produces antibodies and is responsible for the immune system
macrophage: is a type of phagocyte, which is a cell responsible for detecting, engulfing and destroying pathogens