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?

α1 and α2 (B) Signup and view all the answers

Which cells would predominantly express MHC class II molecules?

B-cells, macrophages, and dendritic cells (C) Signup and view all the answers

Where is the MHC locus located in humans?

On Chromosome 6 (C) Signup and view all the answers

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

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

Which of the following statements about MHC proteins is incorrect?

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

What is the primary function of MHC class I molecules?

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

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

Class II MHC (B) Signup and view all the answers

What role do class III MHC molecules primarily serve?

Encode immune proteins like complement components (D) Signup and view all the answers

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

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

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

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

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

Extracellular pathogens (D) Signup and view all the answers

What are super antigens known for in the immune response?

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

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

Activation of cytotoxic T cells (D) Signup and view all the answers

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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Description

This quiz explores the Major Histocompatibility Complex (MHC), including the differences between MHC Class I and Class II. Participants will learn about the structure, functions, and importance of MHC proteins in the immune response. Perfect for students studying immunology or related fields.