antigen

Examples of antigen in the following topics:

• Antigens and Antigen Receptors

  • At the molecular level, an antigen is characterized by its ability to be "bound" at the antigen-binding site of an antibody.
  • The distinct molecular surface features of an antigen capable of being bound by an antibody (a.k.a. antigenic determinant).
  • Some antigens start out as exogenous antigens, and later become endogenous.
  • A native antigen is an antigen that is not yet processed by an APC to smaller parts.
  • Antigen specificity is due primarily to the side-chain conformations of the antigen.

• Antigenic Determinants and Processing Pathways

  • The latter can use epitopes to distinguish between different antigens, and only binds to their specific antigen.
  • Epitopes determine how antigen binding and antigen presentation occur.
  • This is why polysaccharides are generally T-independent antigens and proteins are generally T-dependent antigens.
  • The determinants need not be located on the exposed surface of the antigen in its original form, since recognition of the determinant by T cells requires that the antigen be first processed by antigen presenting cells.
  • There are two different pathways for antigen processing:

• 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).

• Enzyme-Linked Immunosorbent Assay (ELISA)

  • Enzyme-linked immunosorbent assay (ELISA) is a method of quantifying an antigen immobilized on a solid surface.
  • The amount of antibody that binds the antigen is proportional to the amount of antigen present, which is determined by spectrophotometrically measuring the conversion of a clear substance to a colored product by the coupled enzyme.
  • Test solutions containing antigen at an unknown concentration are added to the wells and allowed to bind.
  • The antigen serves as bridge, so the more antigen in the test solution, the more enzyme-linked antibody will bind .
  • The concentration of antigens can be inferred from absorbance readings of standard solutions.

• Classes of T Cells

  • They have antigen receptors that are structurally related to antibodies.
  • These structures help recognize antigens only in the form of peptides displayed on the surface of antigen-presenting cells.
  • These include naive T cells that recognize antigens and are activated in peripheral lymphoid organs.
  • Memory T cells are an expanded population of T cells specific for antigens that can respond rapidly to subsequent encounter with that antigen and differentiate into effector cell to eliminate the antigen.
  • T cells promote the killing of cells that have ingested microorganisms and present foreign antigens on their surface.

• Agglutination Reactions

  • Agglutination is the visible expression of the aggregation of antigens and antibodies.
  • Agglutination reactions apply to particulate test antigens that have been conjugated to a carrier.
  • The endpoint of the test is the observation of clumps resulting from that antigen-antibody complex formation.
  • Direct bacterial agglutination uses whole pathogens as a source of antigen.
  • The binding of antibodies to surface antigens on the bacteria results in visible clumps.

• Humoral Immune Response

  • It relies on antigens (which are also often free in the humours) to detect these pathogens.
  • An antibody/antigen interaction may stimulate an immune response.
  • Not every biomolecule is antigenic and not all antigens produce an immune response.
  • 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 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.

• Clonal Selection and T-Cell Differentiation

  • Antigens are selected to form clones of themselves, both memory and effector.
  • Clonal selection assumes that lymphocytes are selected during antigen presentation because they already have receptors for that antigen.
  • In clonal selection, an antigen is presented to many circulating naive B and (via MHC) T cells, and the lymphocytes that match the antigen are selected to form both memory and effector clones of themselves.
  • The theoretical basis of clonal selection is the assumption that lymphocytes bearing an antigen receptor for an antigen exist long before antigen presentation occurs, 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.

• Blood Groups and Blood Types

  • 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.
  • Blood group O individuals do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM antibodies against both A and B antigens.