MHC I, MHC II & the diferences between them?

Practice Question Solving
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I understand the basic concept, but I’m confused on how they are created, presented and which cells present each type.

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Major Histocompatibility Complex (MHC) molecules are essential components of the immune system. They are proteins on the surface of cells that play a crucial role in immune responses by presenting antigenic peptides to T cells. In humans, these molecules are also known as Human Leukocyte Antigens (HLA).

There are two main classes of MHC molecules: MHC Class I and MHC Class II.

MHC Class I

MHC class I molecules are present on the surface of almost all nucleated cells. This means they are not generally found on red blood cells, which lack a nucleus, but are found on other cell types throughout the body.

MHC Class I molecules present endogenous antigens, which are usually derived from proteins inside the cell. This includes proteins from viruses that have infected the cell or from tumours.

During the process of antigen presentation, proteins from inside the cell are broken down into peptides by a cellular machine known as the proteasome. These peptides are then transported into the endoplasmic reticulum, where they bind to MHC class I molecules. This MHC-peptide complex is then transported to the cell surface, where it can be recognised by cytotoxic CD8+ T cells. If the T cell recognises the antigen as foreign, it can kill the infected or cancerous cell.

MHC Class II

MHC Class II molecules, on the other hand, are generally only found on certain immune cells known as antigen-presenting cells (APCs). This includes dendritic cells, macrophages, and B cells.

MHC Class II molecules present exogenous antigens, which are derived from proteins that are outside the cell. This typically includes proteins from bacteria or parasites that have been ingested by the cell.

During the process of antigen presentation, proteins from outside the cell are broken down into peptides within an organelle known as the lysosome. This organelle then fuses with a vesicle containing the MHC Class II molecule, allowing the peptide to bind to the MHC molecule. This MHC-peptide complex is then transported to the cell surface, where it can be recognised by helper CD4+ T cells. If the T cell recognises the antigen as foreign, it can stimulate an immune response, such as antibody production or activation of cytotoxic T cells.

Differences between MHC Class I and II

Here are some key differences between MHC Class I and II:

  1. Cell type: MHC I is found on all nucleated cells, while MHC II is mainly on antigen-presenting cells.
  2. Antigen origin: MHC I presents endogenous antigens (from inside the cell), while MHC II presents exogenous antigens (from outside the cell).
  3. T cell interaction: MHC I interacts with CD8+ T cells, while MHC II interacts with CD4+ T cells.
  4. Antigen processing: MHC I presents antigens that are processed in the cytosol and transported to the endoplasmic reticulum, while MHC II presents antigens that are processed in lysosomes.
  5. Immune response: MHC I typically triggers a cytotoxic response, leading to the destruction of the cell presenting the antigen. MHC II generally stimulates an immune response, such as antibody production or activation of other immune cells.

I hope this clarifies your confusion!

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