Antigen processing

Antigen Processing

Antigen processing is a crucial biological process in the immune system that involves the breakdown of proteins into peptides that can be presented on the cell surface to T cells. This process is essential for the immune system to recognize and respond to pathogens, as well as to monitor the health of cells. There are two main pathways for antigen processing: the MHC class I pathway and the MHC class II pathway, which are named after the Major Histocompatibility Complex (MHC) molecules involved in presenting the processed antigens to T cells.

MHC Class I Pathway

The MHC class I pathway is primarily involved in presenting antigens from intracellular pathogens, such as viruses and some bacteria, to CD8+ T cells (also known as cytotoxic T cells). In this pathway, proteins within the cell are degraded into peptides by the proteasome, a protein complex in the cytoplasm. These peptides are then transported into the endoplasmic reticulum (ER) by a transporter associated with antigen processing (TAP). In the ER, the peptides are loaded onto MHC class I molecules. The MHC class I-peptide complexes are then transported to the cell surface, where they can be recognized by CD8+ T cells.

MHC Class II Pathway

The MHC class II pathway is responsible for presenting antigens from extracellular sources, such as bacteria and fungi, to CD4+ T cells (also known as helper T cells). In this pathway, extracellular proteins are taken up by the cell through phagocytosis or endocytosis and delivered to lysosomes where they are degraded into peptides. Meanwhile, MHC class II molecules are synthesized in the ER and transported to lysosomes via the Golgi apparatus. In the lysosomes, the peptides are loaded onto MHC class II molecules. The MHC class II-peptide complexes are then transported to the cell surface to be recognized by CD4+ T cells.

Cross-Presentation

An additional process known as cross-presentation allows antigens from extracellular sources to be presented by MHC class I molecules. This process is important for the activation of CD8+ T cells against pathogens that do not infect cells directly. Cross-presentation involves the uptake of extracellular antigens, their delivery to the cytosol, and processing through the proteasome and TAP transporter, similar to the MHC class I pathway.

Role in Immune Response

Antigen processing and presentation are key to initiating the immune response. By displaying peptides on MHC molecules, cells can communicate to T cells whether they are healthy or infected with a pathogen. Recognition of a non-self peptide by a T cell receptor leads to T cell activation, proliferation, and the initiation of an immune response aimed at eliminating the infected cell or pathogen.

Clinical Significance

Defects in antigen processing and presentation can lead to immune system disorders, including autoimmune diseases, where the immune system attacks healthy cells, and immunodeficiency, where the immune system is unable to respond effectively to pathogens. Understanding antigen processing is also crucial for the development of vaccines and immunotherapies, as it informs strategies for eliciting specific immune responses against pathogens or cancer cells.