Like all immune cells, T cells have receptors (in this case called T cell receptors) that recognize antigens. Once the antigen is recognized, the T cell becomes activated. Activated T cells are responsible for destroying the pathogen, either directly by killing the infected cell, or indirectly by activating B cells or innate immune cells—this is often called the cell-mediated immune response.
How do T cells recognize antigens?
Unlike other immune cells, T cells are not able to recognize antigens on their own. The antigens have to be presented to them in a way that allows the T cell receptor to recognize them.
Antigens can be presented to T cells in one of two ways: 1) by normal cells that are infected; or 2) by professional antigen presenting cells.
Cells normally break down proteins and make new ones to replace them. Many cellular proteins are broken down into smaller pieces called peptides. Some of these peptides are sent to the surface of the cell along with a major histocompatibility complex (MHC) class I protein. The MHC class I protein displays the peptide on the outside of the cell so it can be recognized by certain T cells. If a cell has been infected by a foreign pathogen, such as a virus that lives inside the cell, viral peptides (antigens) will be displayed on the surface of the infected cell.
Antigen presenting cells
Dendritic cells, macrophages, and B cells are professional antigen presenting cells that process pathogens. When an antigen presenting cell bumps into a pathogen, it brings it inside the cell using a process called phagocytosis (cell eating). Similar to normal cellular proteins, the pathogen is digested into peptides, which are displayed on the surface of the cell together with a MHC class II protein. Like the MHC class I protein, the MHC class II protein displays the peptide (antigen) and interacts with certain T cells.
Antigen - A protein that is recognized by an immune cell receptor and triggers an immune response.
Antigen presenting cell - A specialized immune cell that presents peptides to CD4+ or CD8+ T cells. Peptides are presented by MHC I or MHC II proteins.
B cell - A type of adaptive immune cell. Also called B lymphocyte.
Cell-mediated immune response - Immune response that involves the direct or indirect activation of immune cells.
Immune cells - Specialized white blood cells (also called leukocytes) that fight infection.
Innate immune cells - Nonspecific immune cells. Recognize many pathogens. First responders to infection.
MHC class I protein - Major histocompatibility complex class I protein. Found on the surface of normal cells. Presents peptides to CD8+ T cells.
MHC class II protein - Major histocompatibility complex class II protein. Found on the surface of antigen presenting cells. Presents peptides to CD4+ T cells.
Pathogens - Bacteria and viruses that can cause disease.
Peptide - A small protein fragment consisting of a chain of amino acids.
Receptor - A protein that is located on the surface of a cell and interacts with other proteins. Receptors act as the “locks” that recognize specific pathogen “keys”.
T cell - A type of adaptive immune cell. Also called T lymphocyte.
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