MCAT Content / Immune System / Major Histocompatibility Complex

Major histocompatibility complex

Topic: Immune System

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:

  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 (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

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