self-antigen

Examples of self-antigen in the following topics:

• Hypersensitivities

  • Hypersensitivities are maladaptive immune reactions against harmless antigens (allergies) or against self antigens (autoimmunity).
  • Maladaptive immune responses toward harmless foreign substances or self antigens that occur after tissue sensitization are termed hypersensitivities.
  • Autoimmunity is a type of hypersensitivity to self antigens that affects approximately five percent of the population.
  • Antibodies that inappropriately mark self components as foreign are termed autoantibodies.
  • Antibodies and TCRs may bind self antigens that are structurally similar to pathogen antigens, which the immune receptors first raised.

• Regulating Immune Tolerance

  • Immune tolerance of self and harmless antigens occurs by deleting B and T cells that recognize those antigens, often near mucosal surfaces.
  • The acquired ability to prevent an unnecessary or harmful immune response to a detected foreign substance known not to cause disease or to self-antigens is described as immune tolerance.
  • The primary mechanism for developing immune tolerance to self-antigens occurs during the selection for weakly, self-binding cells during T and B lymphocyte maturation.
  • Any T or B lymphocytes that recognize harmless foreign or "self" antigens are deleted before they can fully mature into immunocompetent cells.
  • There are populations of T cells that suppress the immune response to self-antigens.

• 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.

• Antibody Functions

  • Essentially, the fact that multimeric antibodies can bind many antigens simultaneously balances their slightly-lower-binding strength for each antibody/antigen interaction.
  • Antibodies secreted after binding to one epitope on an antigen may exhibit cross reactivity for the same or similar epitopes on different antigens.
  • Cross reactivity occurs when an antibody binds not to the antigen that elicited its synthesis and secretion, but to a different antigen.
  • Conversely, antibodies raised against pathogenic molecular components that resemble self molecules may incorrectly mark host cells for destruction, causing autoimmune damage.
  • These antibodies may have been initially raised against the nucleic acid of microorganisms, but later cross-reacted with self-antigens.

• Antigen-presenting Cells: B and T cells

  • Adaptive immunity occurs after exposure to an antigen either from a pathogen or a vaccination.
  • On B cells, these receptors contain antibodies, which are responsible for antigen binding .
  • An antibody is specific for one particular antigen; typically, it will not bind to anything else.
  • Unlike B cells, T cells do not directly recognize antigens.
  • Unlike antibodies, which can typically bind one and only one antigen, T cell receptors have more flexibility in their capacity to recognize antigens presented by MHCs.

• Antibody Structure

  • Variations in antibody structure allow great diversity of antigen recognition among different antibodies.
  • This is why there are numerous antibodies that can each recognize a different antigen.
  • The portion of an antigen that is recognized by the antibody is known as the epitope.
  • The pentamer arrangement means that these macromolecules can bind ten identical antigens.
  • This class supports antigen recognition and subsequent maturation of B cells to plasma cells.

• Immunological Memory

  • As B cells and T cells mature into effector cells, a subset of the naïve populations differentiates into B and T memory cells with the same antigen specificities .
  • One reason why the adaptive immune response is delayed is that it takes time for naïve B and T cells with the appropriate antigen specificities to be identified, activated, and proliferate.
  • Vaccination is based on the knowledge that exposure to noninfectious antigens, derived from known pathogens, generates a mild primary immune response .
  • After initially binding an antigen to the B cell receptor (BCR), a B cell internalizes the antigen and presents it on MHC II.
  • A helper T cell recognizes the MHC II–antigen complex and activates the B cell.

• Cell-Mediated Immunity

  • First, an antigen-presenting cell (APC, such as a dendritic cell or a macrophage) detects, engulfs (via phagocytosis in the case of macrophages or by entry of the pathogen of its own accord in the case of dendritic cells), and digests pathogens into hundreds or thousands of antigen fragments.
  • T cells become activated towards a certain antigen once they encounter it displayed on an MHC II.
  • To do so, a T cell will become activated by interacting with an antigen of the infecting cell or virus presented on the MHC II of an APC.
  • To recognize which cells to pursue, TC recognize antigens presented on MHC I complexes, which are present on all nucleated cells.
  • An antigen-presenting cell (APC), such as a macrophage, engulfs a foreign antigen, partially digests it in a lysosome, and then embeds it in an MHC class II molecule for presentation at the cell surface.

• Alternatives to Dominance and Recessiveness

  • However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses.
  • The M and N alleles are expressed in the form of an M or N antigen present on the surface of red blood cells.
  • In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct.

• Cytotoxic T Lymphocytes and Mucosal Surfaces

  • Lymph gathers antigens as it drains from tissues.
  • These antigens are filtered through lymph nodes before the lymph is returned to circulation.
  • Antigen-presenting cells (APCs) in the lymph nodes capture and process antigens, informing nearby lymphocytes about potential pathogens.