MHC Proteins and Their Functions

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Questions and Answers

What is the primary function of MHC molecules in the immune response?

  • To enhance the activity of B-cells in the immune system
  • To produce antibodies against foreign pathogens
  • To present antigens to T-cells for recognition (correct)
  • To directly eliminate pathogens from the body

Which type of MHC molecule is found on all nucleated cells?

  • MHC class I (correct)
  • MHC class III
  • MHC class II
  • MHC class IV

What distinct feature separates MHC class I proteins from MHC class II proteins?

  • Only MHC class II is involved in presenting antigens to Tc cells
  • MHC class I proteins can bind fragments only from extracellular sources
  • MHC class I is composed of equal-sized alpha and beta subunits
  • MHC class II antigens are recognized by TH cells while class I antigens are recognized by Tc cells (correct)

The antigen binding cleft of MHC class I proteins is formed by which domains?

<p>α1 and α2 (B)</p> Signup and view all the answers

Which cells would predominantly express MHC class II molecules?

<p>B-cells, macrophages, and dendritic cells (C)</p> Signup and view all the answers

Where is the MHC locus located in humans?

<p>On Chromosome 6 (C)</p> Signup and view all the answers

What type of antigens do MHC class I proteins typically bind?

<p>Fragments of proteins degraded by the cytosolic pathway (A)</p> Signup and view all the answers

Which of the following statements about MHC proteins is incorrect?

<p>MHC class II proteins are recognized by Tc cells (B)</p> Signup and view all the answers

What is the primary function of MHC class I molecules?

<p>Presentation of endogenous peptide antigens to CD8+ T cells (A)</p> Signup and view all the answers

Which MHC class is predominantly expressed on antigen-presenting cells?

<p>Class II MHC (B)</p> Signup and view all the answers

What role do class III MHC molecules primarily serve?

<p>Encode immune proteins like complement components (D)</p> Signup and view all the answers

Which statement describes the function of MHC molecules in the context of autoimmunity?

<p>MHC molecules maintain self-tolerance by presenting self peptides. (B)</p> Signup and view all the answers

What is the consequence of TAP deficiency in individuals with Bare Lymphocyte Syndrome?

<p>Decreased levels of CD8+ T cells (A)</p> Signup and view all the answers

What type of antigens are normally presented by MHC class II molecules?

<p>Extracellular pathogens (D)</p> Signup and view all the answers

What are super antigens known for in the immune response?

<p>Simultaneously activating all T cells regardless of specificity (A)</p> Signup and view all the answers

What is a major consequence of MHC molecules displaying foreign peptides?

<p>Activation of cytotoxic T cells (D)</p> Signup and view all the answers

Flashcards

Major Histocompatibility Complex (MHC)

A complex of genes located on chromosome 6 in humans that are crucial for the immune system's ability to recognize and respond to foreign invaders.

Innate Immune Cells

Cells that are part of the innate immune system, providing the first line of defense against pathogens.

Adaptive Immune Cells

Cells that are part of the adaptive immune system, responsible for specific and targeted responses to pathogens.

MHC Molecules

Molecules found on the surface of cells that display fragments of proteins (antigens) to T cells, allowing the immune system to recognize and respond to pathogens.

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MHC Class I

Present on all nucleated cells, MHC class I molecules display fragments of proteins from within the cell to cytotoxic T cells (Tc cells).

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MHC Class II

Present on antigen-presenting cells (APCs), MHC class II molecules display fragments of proteins from outside the cell to helper T cells (Th cells).

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Antigen Presentation

The process by which a pathogen is recognized by the immune system, resulting in an immune response.

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Autoimmunity

A condition where the immune system attacks the body's own tissues, resulting in various diseases such as rheumatoid arthritis and lupus.

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MHC locus

A group of genes responsible for encoding the Major Histocompatibility Complex (MHC) molecules.

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MHC class I molecules

Proteins on the surface of almost all cells that display fragments of proteins found inside the cell, primarily to CD8+ T cells.

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MHC class II molecules

Proteins mainly found on antigen-presenting cells (APCs) that display fragments of proteins from outside the cell, primarily to CD4+ T cells.

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Endogenous antigen presentation

A type of antigen presentation where MHC class I molecules display fragments of proteins that originated inside the cell (e.g., from viruses).

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Exogenous antigen presentation

A type of antigen presentation where MHC class II molecules display fragments of proteins that originated outside the cell (e.g., from bacteria).

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Self-tolerance (MHC)

The process of presenting peptides from the cell's own proteins to T cells in the primary lymphoid organs. This helps the immune system learn to tolerate its own proteins and avoid attacking them.

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Bare Lymphocyte Syndrome

A genetic condition resulting from a deficiency in the TAP protein leading to low expression of MHC class I on the surface of lymphocytes. This can lead to a decrease in CD8+ T cells and increased susceptibility to infections, especially during early life.

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Superantigens

Viral or bacterial proteins that directly bind to both the T-cell receptor and the MHC class II molecule. These proteins can activate a large number of T cells, leading to a massive immune response that can be harmful.

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Study Notes

Major Histocompatibility Complex (MHC)

  • MHC proteins are found on all nucleated cells, unique to each individual, and encoded by MHC genes.
  • In humans, the MHC locus is on Chromosome 6, also known as the Human Leukocyte Antigen (HLA) complex.
  • MHC proteins are membrane-bound glycoproteins with extracellular, transmembrane, and cytoplasmic domains.
  • MHC molecules have an antigen-binding cleft located on the extracellular domain.
    • MHC class I: peptide binding cleft formed by α1 and α2 domains.
    • MHC class II: peptide binding cleft formed by α1 and β2 domains.

MHC Class I vs. MHC Class II

Feature MHC Class I MHC Class II
Cell Location All nucleated cells (including platelets) Antigen-Presenting Cells (APCs) like macrophages, B cells, dendritic cells
Antigen Presentation Foreign antigens presented trigger cytotoxic T cells (Tc cells) Foreign antigens presented trigger helper T cells (Th cells)
Structure Large α subunit, small β2-microglobulin α and β subunits of similar size
Antigen Source Fragments of proteins degraded by cytosolic pathway Fragments of proteins degraded by lysosomal/endocytic pathway

MHC Inheritance

  • MHC molecules are encoded by a cluster of genes (MHC locus).
    • MHC class I genes: Encode glycoproteins expressed on nearly all nucleated cells, presenting endogenous peptides to CD8+ T cells.
    • MHC class II genes: Encode glycoproteins mainly expressed on APCs, presenting exogenous peptides to CD4+ T cells.
    • MHC class III genes: Encode proteins with immune functions (e.g., complement system components, inflammation molecules).

MHC Functions

  • Display "self" class I to show healthy cells.
  • Display foreign peptides (class I) to show infected cells, engaging Tc cells.
  • Display "self" peptides (class I & II) to test T cells for autoreactivity and maintain tolerance to self-proteins.
  • Display foreign peptides (class II) to show infection, activating Th cells.

Antigen Presentation

  • Intracellular pathogens presented by MHC class I.
  • Extracellular pathogens presented by MHC class II.

Bare Lymphocyte Syndrome

  • TAP (transporter associated with antigen processing) deficiencies can cause Bare Lymphocyte Syndrome.
  • Individuals with TAP deficiency have lower class I molecules, higher NK cells, and lower CD8+ T cells, leading to frequent bacterial infections.

Non-peptide Antigen Presentation

  • Some non-protein antigens are recognized by T cells (e.g., lipids like mycolic acid from Mycobacterium tuberculosis).
  • Superantigens are viral/bacterial proteins binding T-cell receptors and MHC class II simultaneously, triggering a strong immune response.

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