MHC Class I And Class II Antigen Presentation: Key Players in the Immune System

Antigen presentation by MHC class I & II

~ Structural fit of Ag in MHC class I & II molecule.

Antigen presentation is a vital process in the immune system to combat pathogens such as viruses, bacteria, and cancer cells. During antigen presentation, the cells known as antigen-presenting cells (APCs) process and display pathogen or cancer-derived peptides on their surface using molecules called the major histocompatibility complex (MHC) to trigger an immune response.

There are two classes of MHC proteins that play distinct roles in antigen presentation: MHC class I and MHC class II.

MHC Class I Antigen Presentation:
MHC class I molecules are present on all nucleated cells and are responsible for presenting intracellular antigens to cytotoxic T cells. In this process, endogenous antigens, such as viral or tumor antigens, are processed and presented on the surface of the cell in complex with MHC class I molecules.

The process of MHC class I antigen presentation occurs in the endoplasmic reticulum (ER) of the APC. The antigen is first broken down into small peptides by proteasomes, which are then transported into the ER via the TAP transporter. In the ER, the peptides bind to MHC class I molecules, which are then transported to the cell surface for presentation to cytotoxic T cells. Once T cells are activated, they differentiate into cytotoxic T lymphocytes (CTLs) that recognize and kill infected or cancerous cells.

MHC Class II Antigen Presentation:
MHC class II molecules are present on professional APCs such as dendritic cells, macrophages, and B cells, which can phagocytose exogenous antigens. The main aim of MHC class II antigen presentation is to stimulate helper T cells. In this process, exogenous antigens are processed and presented on the surface of the APC in complex with MHC class II molecules.

The process of MHC class II antigen presentation occurs in phagosomes or endosomes of the APC. The antigens are first phagocytosed by the APC and broken down into smaller peptides by lysosomal enzymes. These peptides are then transported to endosomes, where they bind to MHC class II molecules. The MHC class II-peptide complexes are then transported to the cell surface for presentation to helper T cells. Once activated, helper T cells can help induce differentiation into various active immune cells, such as B cells and cytotoxic T cells.

In summary, MHC class I and II molecules play crucial roles in antigen presentation, enabling the recognition and elimination of pathogens and cancer cells. MHC class I recognizes intracellular antigens, while MHC class II recognizes extracellular antigens, and both help stimulate the appropriate immune response.

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