Biology · MU Pleven

46.Major histocompatibility complex in the immune response.

Explains the major histocompatibility complex as a set of cell surface molecules that present antigenic peptides to immune cells, also known as HLA in humans. It covers its functions in transplantation, autoimmunity and self versus non-self discrimination, then the three MHC gene classes with focus on Classes I and II. Structure, antigen processing pathways, target cells and T cell responses are contrasted, with a summary table comparing the MHC I and II pathways.

Inside: 8 sections · 3 labelled pictures · 1 table · 13 flashcards · 5 min

Topic 46 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 8 sections, about 554 words (5 min of reading), with 13 flashcards.

In this note

  1. Major histocompatibility complex in the immune response.
  2. Major Histocompatibility Complex (MHC)
  3. MHC Gene Family
  4. MHC Class II
  5. MHC Class I
  6. Antigen Processing Pathway
  7. Variability of MHC Molecules
  8. Characteristics of Antigen Processing Pathways

Major Histocompatibility Complex (MHC)

The Major Histocompatibility Complex (MHC) is a set of cell surface molecules encoded by a large gene family present in all vertebrates. These molecules play a central role in immune recognition by presenting antigenic peptides to immune cells.

In humans, MHC molecules are also known as Human Leukocyte Antigens (HLA).

Functions of MHC

MHC molecules perform several essential immune functions:

  • Mediate interactions between white blood cells (leukocytes) and other immune cells or body cells
  • Determine compatibility in organ transplantation
  • Influence susceptibility to autoimmune diseases due to cross-reacting immune responses
  • Enable immune cells to distinguish self from non-self

Antigen Presentation by MHC

Inside every cell, proteins—either self proteins or proteins derived from foreign biological entities—are constantly synthesized and degraded. Fragments of these proteins (epitopes) are displayed on the cell surface bound to MHC molecules.

  • The presented antigen may be self or non-self
  • This presentation allows immune surveillance by T cells

MHC Gene Family

The MHC gene family is divided into three classes:

  • MHC Class I
  • MHC Class II
  • MHC Class III

In immunity, the focus is mainly on MHC Class I and Class II.

Diversity of Antigen Presentation

Diversity in antigen presentation by MHC Class I and II is achieved through:

  1. 1. Polygeny
  2. a. Multiple MHC genes contribute to antigen presentation
  3. 2. Co-dominant expression
  4. a. Both maternal and paternal alleles are expressed simultaneously
  5. 3. Polymorphism
  6. a. A large number of allelic variants exist within a species

Role of MHC in Immunity

  • MHC Class II
  • Mediates immunization to antigen
  • Involved in specific/acquired/adaptive immunity
  • MHC Class I
  • Mediates destruction of host cells displaying antigen
  • Central to cell-mediated immunity

Lymphocytes and MHC Interaction

Lymphocytes include B cells, T cells, and NK cells, which reside in peripheral lymphoid tissues such as lymph nodes and lymphoid follicles.

  • B cells
  • Act specifically
  • Secrete antibodies
  • Do not interact with MHC
  • T cells
  • Act specifically
  • Recognize antigens only when presented by MHC molecules
  • Natural Killer (NK) cells
  • Act innately
  • Monitor MHC Class I expression
  • When MHC I expression is reduced or absent, NK cells induce programmed cell death
  • Important in tumor surveillance

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