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).

• 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.

• Overview of Adaptive Immunity

  • It provides the body with the ability to recognize and remember specific pathogens through their antigens.
  • The recognition of specific "non-self" antigens in the presence of "self" during the process of antigen presentation
  • The antigen for the pathogen is taken up by an antigen-presenting cell (APC), such as a dendritic cell or macrophage, through phagocytosis.
  • Helper T cells activate B cells, which proliferate and produce antibodies specific to the antigen, while cytotoxic T cells destroy pathogens that bear the antigen that was presented to them by the APCs.
  • The unique variable region allows an antibody to recognize its matching antigen

• Lymphocytes

  • Subtype 2 helper T cells present antigens to B cells.
  • Cytotoxic T cells (CD8s) destroy pathogens associated with an antigen.
  • During antigen presentation, antigen-presenting cells first present antigens to T cells.
  • Then mature helper T cells bind their antigen to naive B cells through BCRs.
  • They are specific to the antigen presented to that BCR and rapidly secrete large amounts of antigen-specific antibodies to prevent reinfection if that antigen is detected again.

• Complete Antigens and Haptens

  • Antigens are basic molecules that induce an immune response when detected by immune system cells.
  • Antigens may be either complete or incomplete based on the nuances of their molecule structure.
  • A hapten is essentially an incomplete antigen.
  • A complete antigen is essentially a hapten-carrier adduct.
  • In this case, the hapten acts as the epitope for the antigen, which binds to the antibodies without causing a response.

• Lymphoid Cells

  • T cells and B cells irecognize specific "non-self" antigens during a process known as antigen presentation with MHC class II (usually done by dendritic cells).
  • Once they have received an antigen, the cells become specifically tailored to eliminate and inhibit the pathogens or pathogen-infected cells that express that antigen.
  • Sometimes these lymphocytes react to antigens that aren't harmful (allergy) or will attack antigens expressed from the host's own body (autoimmunity).
  • Cytotoxic T cells (CD8s) destroy pathogens associated with an antigen.
  • The fully differentiated B and T cells are specific to the presented antigen and work to defend the body against pathogens associated with that antigen.

• Clonal Selection and B-Cell Differentiation

  • B cells mature in the bone marrow, where they undergo VDJ recombination to produce unique receptors that do not react to self-antigens.
  • B cells primarily function to make antibodies against antigens, act as antigen-presenting cells (APCs), and eventually develop into memory B cells to provide long-term immunity.
  • However, B cell recognition of antigens is not the only element necessary for B cell activation.
  • T cell independent activation occurs when antigens directly bind to B cell themselves, usually through cross-linking the antigen to the B cell receptor or receiving the antigen with a toll-like receptor.
  • Clonal selection is a theory stating that B cells express antigen-specific receptors before antigens are ever encountered in the body.