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Antigen presentation is when cells digest foreign proteins and present them on their surface to other cells, triggering an immune response. T lymphocytes recognize foreign antigens through a T cell receptor and major histocompatibility complex (MHC) molecules. MHC class I molecules present endogenous antigens, while MHC class II molecules present exogenous antigens. Antigen-presenting cells include dendritic cells, macrophages, and B lymphocytes.
Antigen presentation is one aspect of the immune response. In it, the body’s cells digest foreign proteins or antigens into small peptides and express them on their surface. These peptides are incorporated into the cell membrane and are presented to other cells which can generate an immune response. Foreign proteins such as bacteria and viruses are mainly those that are degraded during antigen presentation.
The immune response is the body’s way of fighting disease. Various cell types are involved in this response. Cytotoxic T lymphocytes are activated to attack and destroy virus-infected cells. T helper cells secrete proteins known as cytokines when activated, and these cytokines recruit other cells to the site of infection. In order for cytotoxic T cells or helper T cells to mount a response, they must have foreign antigens presented to them by other cell types.
T lymphocytes have a molecule on their surface called a T cell receptor. When this T cell receptor binds to an antigen on the surface of other cells, the T cell is triggered 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. MHC class I molecules are present on all cells that have a nucleus. Both MHC class I and class II 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.
MHC class I molecules bind to endogenous antigens within 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 MHC class I molecule and are transported to the surface for presentation to cytotoxic T lymphocytes. Cytotoxic T lymphocytes can then mount an attack against the virus-infected cell.
MHC class II 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 in a vesicle and moves to the cell surface during antigen presentation to T helper lymphocytes. These helper T lymphocytes secrete cytokines that recruit other cell types to the site of infection.
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