Positive selection

(noun)

The process by which T cells are screened so that only those capable of binding to MHC are kept alive.

Related Terms

Examples of Positive selection in the following topics:

  • Maturation of T Cells

    • T cells originate from hematopoietic stem cells in the bone marrow and undergo positive and negative selection in the thymus to mature.
    • Positive selection designates T cells capable of interacting with MHC.
    • A thymocyte's differentiation into either a helper or cytotoxic version is also determined during positive selection.
    • Double-positive cells (CD4+/CD8+) that are positively selected on MHC class II molecules will eventually become CD4+ helper T cells, while cells positively selected on MHC class I molecules mature into CD8+ cytotoxic T cells.
    • Thymocytes that survive positive selection migrate towards the boundary of the thymic cortex and thymic medulla (the part of the thymus where T cells enter circulation).

  • Clonal Selection and Tolerance

    • Clonal selection and tolerance select for survival of lymphocytes that will protect the host from foreign antigens.
    • The preservation of useful specificities is called positive selection.
    • Positive selection ensures maturation of T cells whose receptors bind weakly to self major histocompatibility complex molecules.
    • clonal selection of the B and T lymphocytes:1.
    • Describe the importance of central and peripheral tolerance and distinguish between positive and negative clonal selection

  • Frequency-Dependent Selection

    • In frequency-dependent selection, phenotypes that are either common or rare are favored through natural selection.
    • Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).
    • An example of positive frequency-dependent selection is the mimicry of the warning coloration of dangerous species of animals by other species that are harmless.
    • Negative frequency-dependent selection serves to increase the population's genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.
    • Positive frequency-dependent selection reinforces the common phenotype because predators avoid the distinct coloration.

  • Employee Selection

    • Selection is the process—based on filtering techniques that ensure added value—of choosing a qualified candidate for a position.
    • However, it most commonly refers to the selection of workers.
    • A selection system should depend on job analysis.
    • This ensures that the selection criteria are job related and propose value additions for the organization.
    • Break down the human resource selection process as organizations pursue new employee talent

  • Shared Features of Bacteria and Archaea

    • In phylogenetic trees based upon different gene/protein sequences of prokaryotic homologs, the archaeal homologs are more closely related to those of Gram-positive bacteria.
    • Archaea and Gram-positive bacteria also share conserved indels in a number of important proteins, such as Hsp70 and glutamine synthetase I.
    • Gupta has proposed that the Archaea evolved from Gram-positive bacteria in response to antibiotic selection pressure.
    • His proposal is that the selective pressure towards resistance generated by the Gram-positive antibiotics was eventually sufficient to cause extensive changes in many of the antibiotics' target genes, and that these strains represented the common ancestors of present-day Archaea.
    • The evolution of Archaea in response to antibiotic selection, or any other competitive selective pressure, could also explain their adaptation to extreme environments (such as high temperature or acidity) as the result of a search for unoccupied niches to escape from antibiotic-producing organisms; Cavalier-Smith has made a similar suggestion.

  • Selective and Differential Media

    • Selective media are used for the growth of only selected microorganisms.
    • Selective growth media for eukaryotic cells commonly contain neomycin to select cells that have been successfully transfected with a plasmid carrying the neomycin resistance gene as a marker.
    • Some examples of selective media include:
    • Eosin methylene blue (EMB) that contains methylene blue – toxic to Gram-positive bacteria, allowing only the growth of Gram negative bacteria.
    • Mannitol salt agar (MSA) which is selective for Gram-positive bacteria and differential for mannitol.

  • The Psychology of Recruiting and Selecting Employees

    • Therefore, it is beneficial to attract not just a large quantity of applicants, but a group of individuals with the necessary skills for the position.
    • To find the best fit, managers create a list of relevant criteria composed of critical skills, behaviors, and attitudes for each position.
    • Industrial and organizational (I–O) psychologists use a variety of measures to select applicants who are the best fit for a position.
    • Another tool used for selection is personality testing.
    • As an example, let's look at a job interview for a position as a banker.

  • Antibiotics and Selective Toxicity

    • Antibiotics are able to selectively target specific types of bacteria without harming the infected host.
    • He then proposed the idea that it might be possible to create chemicals that would act as a selective drug that would bind to and kill bacteria without harming the human host.
    • More specifically, narrow spectrum antibiotics target specific types of bacteria, such as Gram-negative or Gram-positive bacteria, whereas broad spectrum antibiotics affect a wide range of bacteria.
    • The remainder must be tested for their selective toxicities and therapeutic activities, and the best candidates can be examined and possibly modified.
    • In antibacterial production, microorganisms must be isolated, cultured, and tested for growth inhibition of target organisms and for their selective toxicity.

  • Routing

    • Routing is the process of selecting paths in a network along which to send network traffic.
    • Routing is the process of selecting paths in a network along which to send network traffic.
    • GPS, or global positioning system, is a technological advancement that has helped companies determine which routes are the most expensive to maintain.

  • Random Sampling

    • An online poll in which a person is asked to given their opinion about something is not random because only those people with strong opinions, either positive or negative, are likely to respond.
    • Some of these clusters are randomly selected.
    • Then, all the individuals in the chosen cluster are selected to be in the sample.
    • For example, while surveying households within a city, we might choose to select 100 city blocks and then interview every household within the selected blocks, rather than interview random households spread out over the entire city.
    • Online and phone-in polls also produce biased samples because the respondents are self-selected.