Antigen recognition and processing are critical processes in the adaptive immune response, which enables the immune system to specifically recognize and respond to a wide range of foreign substances, including pathogens and cancer cells.
Antigens are molecules that can be recognized by the immune system as foreign or non-self. Antigens can be proteins, carbohydrates, lipids, or nucleic acids, and they can come from a variety of sources, including pathogens, cancer cells, and environmental substances. Antigens are recognized by specific receptors on immune cells called antigen receptors. In B cells, the antigen receptor is a membrane-bound antibody molecule, while in T cells, the antigen receptor is a T cell receptor (TCR).
Antigen processing is the process by which antigens are broken down into smaller peptides and presented on the surface of antigen-presenting cells (APCs) in association with major histocompatibility complex (MHC) molecules. MHC molecules are highly polymorphic cell surface glycoproteins that are involved in the recognition of self and non-self by the immune system. MHC molecules are divided into two classes: MHC class I molecules, which are expressed on the surface of all nucleated cells and present peptides derived from intracellular proteins, and MHC class II molecules, which are expressed only on the surface of professional APCs such as dendritic cells, macrophages, and B cells, and present peptides derived from extracellular proteins.
Antigen-presenting cells process antigens by endocytosis, phagocytosis, or pinocytosis, and degrade them into smaller peptides in lysosomes. The resulting peptides are then loaded onto MHC molecules in the endoplasmic reticulum (ER) or endocytic vesicles, and transported to the cell surface for presentation to T cells. T cells can recognize only antigenic peptides presented in association with MHC molecules. CD8+ T cells recognize antigenic peptides presented on MHC class I molecules, while CD4+ T cells recognize antigenic peptides presented on MHC class II molecules.
Antigen recognition and processing are critical for the initiation and regulation of adaptive immune responses. Defects in antigen recognition and processing can lead to immune dysfunction and disease, such as immunodeficiency, autoimmunity, and cancer. Therefore, understanding the mechanisms of antigen recognition and processing is essential for the development of effective vaccines and immunotherapies.