self-antigen

Examples of self-antigen in the following topics:

• Clonal Selection and Tolerance

  • Clonal selection occurs after immature lymphocytes express antigen receptors.
  • The cells with useful receptors are preserved, and many potentially harmful, self antigen-reactive cells are eliminated by processes of selection induced by antigen receptor engagement .
  • Negative selection is the process that eliminates developing lymphocytes whose antigen receptors bind strongly to self antigens present in the lymphoid organs.
  • "Self"-antigens from the body's own tissues 4.
  • Foreign antigen 6.

• Immunoassays for Disease

  • Immunoassays are laboratory techniques based on the detection of antibody production in response to foreign antigens.
  • The function of the immune system is to recognize self antigens from non-self antigens and defend the body against non-self (foreign) agents.
  • These biochemical and serological techniques are based on the detection and quantitation of antibodies generated against an infectious agent, a microbe, or non-microbial antigen.
  • Most assays rely on the formation of large immune complexes when an antibody binds to a specific antigen which can be detected in solution or in gels.
  • Recent methods employ pure antibodies or antigens that have been immobilized on a platform and that can be measured using an indicator molecule.

• Antigens and Antigen Receptors

  • "Self" antigens are usually tolerated by the immune system; whereas "non-self" antigens can be identified as invaders and can be attacked by the immune system.
  • Non-microbial exogenous (non-self) antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.
  • Some antigens start out as exogenous antigens, and later become endogenous.
  • In order to keep the cytotoxic cells from killing cells just for presenting self-proteins, self-reactive T cells are deleted from the repertoire as a result of tolerance (also known as negative selection).
  • Antigen specificity is due primarily to the side-chain conformations of the antigen.

• Type II (Cytotoxic) Reactions

  • The antigens recognized in this way may either be intrinsic ("self" antigen, innately part of the patient's cells) or extrinsic (adsorbed onto the cells during exposure to some foreign antigen, possibly as part of infection with a pathogen).
  • This causes a B cell response, wherein antibodies are produced against the foreign antigen.
  • IgG and IgM antibodies bind to these antigens to form complexes that activate the classical pathway of complement activation to eliminate cells presenting foreign antigens (which are usually, but not in this case, pathogens).
  • Here, cells exhibiting the foreign antigen are tagged with antibodies (IgG or IgM).
  • They differ from hypersensitivity reactions in that the antigens driving the immune process are self-antigens rather than non-self as in hypersensitivity reactions.

• Antigen-Presenting Cells

  • Antigen presentation is a process where immune cells capture antigens and then enable their recognition by T-cells.
  • The host's cells express "self" antigens that identify them as such.
  • These antigens are different from those in bacteria ("non-self" antigens) or in virally-infected host cells ("missing-self").
  • Unlike B cells, T cells fail to recognize antigens in the absence of antigen presentation, with the important exception of the superantigens.
  • In the upper pathway; foreign protein or antigen (1) is taken up by an antigen-presenting cell (2).

• Humoral Immune Response

  • An antibody/antigen interaction may stimulate an immune response.
  • Not every biomolecule is antigenic and not all antigens produce an immune response.
  • B cells containing antibodies that recognize "self" antigens are destroyed before they can mature, preventing the immune system from attacking the host.
  • The antigen-antibody complex stimulates the complement system described previously, destroying the cell bearing the antigen.
  • B cell receptors, containing antibodies (termed antigen-binding site in the picture) are embedded in the membranes of B cells and bind a variety of antigens through their variable regions.

• Clonal Selection and T-Cell Differentiation

  • Antigens are "selected" to form clones of themselves, both memory and effector.
  • Clonal selection assumes that lymphocytes already have receptors for that antigen, and are selected during antigen presentation because they already have that unique antigen receptor.
  • The theoretical basis of clonal selection is the assumption that lymphocytes bearing an antigen receptor for an antigen exist long before antigen presentation occurs, and is explained by the idea of random mutations (VDJ recombination) that occur during lymphocyte maturation.
  • During antigen presentation, pre-existing lymphocytes that bear that antigen receptor are merely selected because they can bind with that antigen.
  • This assumes that by random mutations resulted in lymphocytes that were auto-reactive, instead of reactive to non-self antigens.

• Lymphocytes

  • Subtype 2 helper T cells present antigens to B cells.
  • Suppressor T cells (T-reg cells)- T cells that kept some of their ability to bind to self-cells.
  • While in the bone marrow, B cells are sorted through positive and negative selection in  a manner similar but different from T cell maturation in the thymus, with the same process of killing B cells that are either nonreactive to antigens or reactive to self antigens.
  • During antigen presentation, antigen presenting cells first present antigens to T cells.
  • They rapidly secrete large amounts of antigen specific antibodies should that antigen be detected in the body again, which prevents reinfection by that antigen.

• Blood Groups and Blood Types

  • Red blood cells have surface-expressed proteins that define the self/not-self nature of the cells.
  • If an individual is exposed to a blood group antigen (A or B) that is not recognized as self, the individual can become sensitized to that antigen.
  • This will cause the immune system to make specific antibodies to a particular blood group antigen and form an immunological memory against that antigen.
  • Blood group A individuals have the A antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the B antigen.
  • Blood group B individuals have the B antigen on their surface of their RBCs, and blood serum containing IgM antibodies against the A antigen.

• Antigenic Determinants and Processing Pathways

  • An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, T cells which can use epitopes to distinguish between different antigens, and only bind to the antigen that they are specific to.
  • Although epitopes are usually thought to be derived from non-self proteins, sequences derived from the host that can be recognized are also classified as epitopes.
  • Epitopes determine how antigen binding and antigen presentation occurs.
  • This is why polysaccharides are generally T-independent antigens and proteins are generally T-dependent antigens.
  • In order for an antigen presenting cell (APC) to present an antigen to a naive T cell, it must first be processed into a form in which the antigenic determinant can be recognized by the T cell receptor.