Antigen presentation is one aspect of the immune response. In it, cells of the body digest foreign proteins or antigens into small peptides and express them on their surface. These peptides are embedded in the cell membrane and are presented to other cells that can generate an immune response. Foreign proteins such as bacteria and viruses are primarily those that get degraded during antigen presentation.
The immune response is the body’s way of fighting off disease. Various cell types are involved in this response. Cytotoxic T-lymphocytes are activated to attack and destroy virus-infected cells. Helper T-lymphocytes secrete proteins known as cytokines when activated, and these cytokines recruit other cells to the site of infection. In order for either cytotoxic T-lymphocytes or helper T-lymphocytes to mount a response, they need to have foreign antigens presented to them by other cell types.
T-lymphocytes have a molecule on their surface called the T-cell receptor. When this T-cell receptor binds to an antigen on the surface of other cells, the T-cell is activated to respond. These T-lymphocytes must be able to distinguish foreign pathogens from self proteins. The T-cell can recognize an antigen as foreign only when it is associated with a major histocompatibility complex (MHC) or self antigen.
There are two classes of MHC molecules. Class I MHC molecules are present on all cells that have a nucleus. Both Class I and Class II MHC molecules are present on the surface of specialized cells involved in antigen presentation. These cells, known as antigen-presenting cells, include dendritic cells, macrophages, and B-lymphocytes.
Class I MHC molecules bind to endogenous antigens inside the cell. Endogenous antigens, which include viral proteins produced after a cell has been infected, are then digested into small peptides by enzymes in the cytoplasm. These peptides bind to the Class I MHC molecule and are carried to the surface for presentation to cytotoxic T-lymphocytes. The cytotoxic T-lymphocytes can then mount an attack against the virus-infected cell.
Class II MHC molecules bind to exogenous antigens that come from outside the cell. Exogenous antigens include bacteria and toxins and these antigens are engulfed by the antigen presenting cell. Once inside the cell, these antigens are digested by enzymes and combined with the Class II MHC molecule. This complex is packaged into a vesicle and moves to the cell surface during antigen presentation to helper T-lymphocytes. These helper T-lymphocytes secrete cytokines that recruit other cell types to the site of infection.