antigenic drift

Examples of antigenic drift in the following topics:

• Emergence of Viral Pathogens

  • One common evolutionary process whereby viral genes change over time is called genetic drift, where individual bases in the DNA or RNA mutate to other bases.
  • Two processes drive the antigens to change: antigenic drift and antigenic shift (antigenic drift being the more common).
  • Antigenic drift is a mechanism for variation by viruses that involves the accumulation of mutations within the antibody-binding sites so that the resulting viruses cannot be inhibited as well by antibodies against previous strains, making it easier for them to spread throughout a partially immune population.
  • Antigenic drift occurs in both influenza A and influenza B viruses.
  • The rate of antigenic drift is dependent on two characteristics: the duration of the epidemic and the strength of host immunity.

• 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.

• 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.

• 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.

• T Cell Receptors

  • The T Cell Receptor (TCR) found on the surface of T cells is responsible for recognizing antigens.
  • MHC molecules and peptides form complexes on the surface of antigen presenting cells (APCs).
  • The biochemical signals that are triggered in T cells following antigen recognition are transduced not by the TCR itself, but by invariant proteins (CD3, and zeta), which are non-covalently linked to the antigen receptor to form the TCR complex.
  • T cells also express other membrane receptors that do not recognize antigens but participate in responses to antigens; these are collectively called 'accessory molecules'.
  • The antigen receptor of MHC-restricted CD4 helper T cells and CD8 cytolytic T cell is a heterodimer consisting of two transmembrane polypeptide chains, designated alpha and beta, covalently linked to each other by disulfide bonds.

• Dendritic Cells

  • Dendritic cells are immune cells that function to process antigens and present them to T cells.
  • The function of epithelial dendritic cells is to capture microbial protein antigens and to transport the antigens to draining lymph nodes.
  • During their migration to the lymph nodes, the dendritic cells mature to become extremely efficient at presenting antigens and stimulating naive T cells, hence their classification as antigen presenting cells.
  • Mature dendritic cells reside in the T cell zones of the lymph nodes, and in this location they display antigens to T cells.
  • The ultimate consequence is priming and activation of the immune system for attack against the antigens which the dendritic cell presents on its surface.

• Complement Fixation

  • Complement subsequently binds to this antigen-antibody complex formed and will cause the red blood cells to lyse .
  • The second component is a known antigen and patient serum added to a suspension of sheep red blood cells in addition to complement.
  • Patient serum is first added to the known antigen, and complement is added to the solution.
  • If the serum contains antibody to the antigen, the resulting antigen-antibody complexes will bind all of the complement.
  • If complement has not been bound by an antigen-antibody complex formed from the patient serum and known antigens, it is available to bind to the indicator system of sheep cells and anti-sheep antibody.

• 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.