own proteins ( self-tolerance ). However, sometimes the immune system fails to distinguish “self” from “non-self”. This loss of self-recognition & or self-tolerance leads to autoimmune disease. The body’s immune system has the ability to recognize and bind to at least a billion different epitopes. Before a particular antigen even enters the body, T cells and B cells that recognize and respond to that invader are waiting and ready. Cells of the immune system can even recognize artificially made molecules that do not exist in nature. The diversity of antigen receptors in both B cells and T cells is the result of shuffling and rearranging a few hundred versions of several small gene segments. This process is called Genetic Recombination . The gene segments are put together in different combinations as the lymphocytes are developing from stem cells. Because of this, each B cell or T cell has a unique set of gene segments that code for its unique antigen receptor.
Major Histocompatibility Complex Antigens (MHC)
In the plasma membrane of body cells are “self” antigens, called MHC antigens. 1,000s to 100,000s of MHC molecules mark the surface of each of your body cells except RBCs. MHC antigens are the reason that tissues may be rejected when they are transplanted from one person to another. The major function of an MHC molecule is to help T cells recognize that an antigen is foreign, not “self”. 2 types of MHC antigens: Class I MHC (MHC-I) molecules are built into the plasma membrane of all body cells except RBCs. Class II MHC (MHC-II) molecules appear on the surface of antigen-presenting cells (described later).
Pathways of Antigen Processing
For an immune response to occur, B and T cells must recognize and bind to antigens in lymph, interstitial fluid, or blood plasma. T cells (Helper or Cytotoxic) only recognize fragments of antigenic proteins that are processed and presented in a certain way.
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