Antigen processing and presentation is the process by which antigens are processed and presented to T cells by antigen-presenting cells (APCs). This is a critical step in the adaptive immune response, as it allows T cells to recognize and respond to specific antigens.
APCs include dendritic cells, macrophages, and B cells, which all have the ability to take up and process antigens. Once an antigen is internalized, it is broken down into smaller peptide fragments by proteases in the endocytic or lysosomal compartments of the cell. These peptide fragments are then loaded onto major histocompatibility complex (MHC) molecules.
MHC molecules are highly polymorphic cell surface proteins that are divided into two classes, MHC class I and MHC class II. MHC class I molecules are expressed on all nucleated cells and present antigens to CD8+ T cells, also known as cytotoxic T cells. MHC class II molecules are primarily expressed on APCs and present antigens to CD4+ T cells, also known as helper T cells.
Once an antigen is loaded onto an MHC molecule, the MHC-antigen complex is transported to the cell surface where it is displayed to T cells. CD8+ T cells recognize antigens presented on MHC class I molecules and can directly kill infected or abnormal cells. CD4+ T cells recognize antigens presented on MHC class II molecules and provide help to other immune cells, such as B cells and CD8+ T cells.
Antigen processing and presentation is a highly regulated process, and defects in this process can lead to immune dysfunction and disease. For example, defects in MHC class I processing and presentation can lead to increased susceptibility to viral infections and tumors, while defects in MHC class II processing and presentation can lead to autoimmune diseases such as rheumatoid arthritis and multiple sclerosis.