Antigen Presentation: MHC Class I vs. MHC Class II

Summary

This video explains the immune system's remarkable ability to distinguish between self and nonself through antigen presentation. It covers the two main pathways: MHC class I and MHC class II. MHC class I presents endogenous antigens from inside the cell, such as viral or cancerous components, to CD8 T cells. MHC class II presents exogenous antigens, like extracellular bacteria, to CD4 T helper cells. The process involves complex interactions between molecules, proteins, and cellular structures to ensure the immune system can recognize and respond to potential threats effectively.

Takeaways

• 😀 The immune system distinguishes self from nonself to protect the body from pathogens like viruses.
• 😀 Antigens are presented to immune cells to trigger an immune response when a specific antigen is recognized.
• 😀 MHC class I and MHC class II are the two protein classes responsible for presenting antigens on the cell surface.
• 😀 Cytosolic proteins are degraded into small peptides (about 15 amino acids) by the proteasome.
• 😀 These peptides are delivered to the endoplasmic reticulum by the TAP transporter, where they bind to MHC class I molecules.
• 😀 When the MHC class I molecule binds to a peptide, it dissociates from the TAP transporter and helper proteins.
• 😀 The antigen-loaded MHC class I molecule is transported via vesicle to the plasma membrane for presentation to CD8 T cells.
• 😀 MHC class II molecules present exogenous antigens, which are taken up by cells through endocytosis.
• 😀 In the endosome, a small molecule initially blocks the binding site of MHC class II molecules, which is later replaced by the antigen.
• 😀 The antigen-MHC class II complex is transported to the plasma membrane, where it is recognized by CD4 T helper cells.
• 😀 MHC class I antigens are derived from inside the cell (endogenous), including normal self-antigens and viral or cancerous components.
• 😀 MHC class II antigens are acquired from outside the cell (exogenous), such as extracellular bacteria.

Q & A

What is the key feature of the immune system discussed in the script?

The key feature is the immune system's ability to distinguish self from nonself, or foreign, which is crucial for recognizing and defending against pathogens like viruses.

How does the immune system recognize pathogens?

The immune system recognizes pathogens through the presentation of antigens to immune cells. Once an immune cell identifies its specific antigen, an immune response is triggered.

What are the two protein classes involved in antigen presentation?

The two protein classes involved in antigen presentation are MHC class I and MHC class II molecules.

How are cytosolic proteins processed and presented by MHC class I molecules?

Cytosolic proteins are broken down into small peptides by the proteasome, then transported to the endoplasmic reticulum via the TAP transporter. The peptides bind to MHC class I molecules, which then move to the cell surface for presentation.

What happens when the MHC class I molecule binds to a peptide?

When the MHC class I molecule binds to a peptide, the TAP transporter and helper proteins dissociate. The MHC class I molecule with the peptide then travels to the cell surface for antigen presentation.

What is the role of effector CD8 T cells in the immune response?

Effector CD8 T cells recognize the antigen-MHC class I complex on the cell surface and are involved in the immune response, especially in targeting infected or abnormal cells.

How are exogenous antigens processed by MHC class II molecules?

Exogenous antigens are taken up by cells via endocytosis, interact with MHC class II molecules in the endosome, and are presented on the cell surface after the vesicle fuses with the plasma membrane.

What happens to the MHC class II molecules during antigen presentation?

MHC class II molecules have their binding site initially blocked by a small molecule. However, another helper protein in the endosome facilitates the exchange of this small molecule with an antigen, allowing presentation on the cell surface.

What are the differences between MHC class I and class II antigen presentation?

MHC class I presents endogenous antigens, usually derived from inside the cell, such as self-antigens, neoantigens from cancer cells, or viral components. MHC class II presents exogenous antigens, typically from external sources like extracellular bacteria.

What is the significance of the MHC class I and class II pathways in immune defense?

MHC class I and II pathways are essential for the immune system to identify and respond to different types of threats. Class I deals with internal threats (like viral infections and cancer cells), while class II addresses external threats (like bacteria).