Antigen Processing : antigenic proteins are broken down into peptide fragments within the cell that then associate with MHC molecules creating an Antigen-MHC complex which is inserted into the plasma membrane of a body cell. This insertion is called Antigen Presentation . If the peptide fragment comes from a “self” protein, T cells ignore the antigen-MHC complex. However, if the peptide fragment comes from a foreign protein, T cells recognize the antigen-MHC complex as an intruder, and an immune response takes place. Antigen processing and presentation occurs in 2 ways depending on whether the antigen is located outside (Exogenous) or inside (Endogenous) body cells:
1) Processing of Exogenous Antigens
Exogenous Antigens are foreign antigens present in fluids OUTSIDE body cells. These include bacteria, bacterial toxins, parasitic worms, inhaled pollen and dust, and viruses that have not yet infected a body. Antigen-Presenting Cells (APCs) process and present exogenous antigens. APCs include dendrite cells, macrophages, B cells. They are strategically located in regions of the body where antigens are likely to penetrate innate defenses such as epidermis and dermis, mucous membranes lining the respiratory tract, GI tract, urinary tract, and reproductive tracts, and lymph nodes. After exocytosis and processing of an antigen, the APC migrates to lymphatic tissue to present the antigen to Cytotoxic T cells. The presentation of an exogenous antigen together with MHC-II molecules by APCs informs Cytotoxic T cells that intruders are present in the body and that an immune action should begin.
2) Processing of Endogenous Antigens
Endogenous Antigens are foreign antigens present INSIDE body cells. These include viral proteins produced after a virus infects the cell and takes over the cells metabolic activity, toxins produced from intracellular bacteria, or abnormal proteins synthesized by a cancerous cell. These antigens are SELF antigens and do not come from outside the body. The display of an endogenous antigen bound to an MHC-I molecule signals that a cell has been infected and need assistance.
Clonal Selection
When a specific antigen invades the body, there are usually many copies of that antigen spread throughout the body’s tissues possibly from previous encounters. Often these copies initially outnumber the small group of helper T cells, cytotoxic T cells, and B cells. So, when each of these lymphocytes encounters a copy of the antigen and receives stimulatory cues, it undergoes Clonal Selection . This is the process by which lymphocytes proliferate (divides several times) and differentiate forming a more highly specialized cells in response to a specific antigen.
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