Understanding Antigen Presentation: Role of Antigen Presenting Cells (APCs) and MHC Molecules

bind antigens or specific peptides presented by antigen presenting cells

The process we are referring to is known as antigen presentation

The process we are referring to is known as antigen presentation. Antigen presenting cells (APCs), such as macrophages, dendritic cells, and B cells, play a crucial role in the immune response by capturing, processing, and presenting antigens to T cells.

APCs have specialized cell surface proteins called major histocompatibility complex (MHC) molecules, also known as human leukocyte antigens (HLAs) in humans. These MHC molecules exist in two classes – MHC class I and MHC class II. Each class of MHC molecule has a different role in antigen presentation.

MHC class I molecules are present on nearly all nucleated cells and are responsible for presenting intracellular peptides. These peptides are derived from proteins synthesized within the cell, typically through viral or tumor-associated proteins. The peptides are generated in the cytoplasm and 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 for recognition by CD8+ T cells, also known as cytotoxic T cells. If the T cell receptor (TCR) on a specific cytotoxic T cell recognizes the antigen presented by MHC class I, it triggers an immune response, leading to the elimination of the infected or cancerous cell.

On the other hand, MHC class II molecules are expressed predominantly on APCs and are responsible for presenting extracellular antigens. These can be peptide fragments from proteins that have been taken up by the APCs through phagocytosis or pinocytosis. The antigens are then degraded within endosomes, and the resulting peptides bind to MHC class II molecules within the endosomal compartments. The MHC class II-peptide complexes are subsequently transported to the cell surface, where they can be recognized by CD4+ T cells, also known as helper T cells. The interaction between the TCR on a specific helper T cell and the antigen-MHC class II complex leads to the activation of the immune response, such as activating B cells or cytotoxic T cells, or releasing cytokines to regulate the immune system.

Both MHC class I and MHC class II molecules have specific binding sites that allow them to bind antigens or specific peptides. MHC molecules have a highly polymorphic nature, meaning they have different variants across individuals. This polymorphism allows for a wide range of antigens to be presented to T cells, thereby increasing the likelihood of successful immune responses against varied pathogens.

In summary, antigen presenting cells play a crucial role in the immune response by capturing and processing antigens, then presenting specific peptides on MHC molecules. This allows for the recognition of antigens by T cells, which initiates the activation of the immune system to fight against infections, cancer, and other diseases.

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