antigenic drift

(noun)

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.

Related Terms

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.

  • Population Genetics

    • A human with a type IA allele will display A-type proteins (antigens) on the surface of their red blood cells.
    • When allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies, the phenomenon is called genetic drift.
    • Genetic drift and natural selection usually occur simultaneously in populations, but the cause of the frequency change is often impossible to determine.
    • Together, the forces of natural selection, genetic drift, and founder effect can lead to significant changes in the gene pool of a population.
    • In humans, each blood type corresponds to a combination of two alleles, which represent a the type of antigens displayed on the outside of a red blood cell.

  • 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

    • 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.
    • 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.
    • 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.
    • 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.

  • 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).

  • Genetic Drift

    • Genetic drift is the change in allele frequencies of a population due to random chance events, such as natural disasters.
    • Genetic drift is the converse of natural selection.
    • Small populations are more susceptible to the forces of genetic drift.
    • Thus even while genetic drift is a random, directionless process, it acts to eliminate genetic variation over time.
    • Genetic drift in a population can lead to the elimination of an allele from that population by chance.

  • A Microscopic View: Drift Speed

    • The drift velocity is the average velocity that a particle achieves due to an electric field.
    • The drift velocity vdis the average velocity of the free charges after applying the field.
    • The drift velocity is quite small, since there are so many free charges.
    • The carriers of the current each have charges q and move with a drift velocity of magnitude vd.
    • Relate the drift velocity with the velocity of free charges in conductors

  • 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.