Positive Selection
Positive Selection - Related Videos
14:29
Examination of Thymic Positive and Negative Selection by Flow Cytometry
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Cited by 22 •
2012
We present a flow cytometry-based method to examine T cell development in vivo using genetically manipulated mice on a wildtype or T cell receptor transgenic background.
03:14
Artificial Selection - Concepts
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2019
Natural Selection and Adaptive Evolution In natural settings, adaptive evolution takes place by natural selection, a process in which differences in traits lead to differences in survival and reproductive success between individuals. This happens because individuals with certain phenotypes have a higher probability of survival and/or produce more surviving offspring than individuals with other phenotypes. A classic example of this was demonstrated by Rosemary and Peter Grant studying the...
02:32
Position-effect Variegation
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2020
In 1928, a German botanist Emil Heitz observed the moss nuclei with a DNA binding dye. He observed that while some chromatin regions decondense and spread out in the interphase nucleus, others do not. He termed them euchromatin and heterochromatin, respectively. He proposed that the heterochromatin regions reflect a functionally inactive state of the genome. It was later confirmed that heterochromatin is transcriptionally repressed, and euchromatin is transcriptionally active chromatin.
06:48
Natural Selection - Concepts
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2019
Fitness Widespread variation of phenotypes in natural populations provides the raw material for evolution, which is the change in the inherited traits of populations over successive generations. Natural selection is one of the main mechanisms of evolution and requires variable traits to be heritable and associated with differential survival and/or reproductive success. Phenotypes that correlate with greater success will have more offspring that survive to reproduce in the next generation, and...
08:53
Natural Selection - Student Protocol
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2019
Simulating Natural Selection ExpandTo begin, cut the white, black, and gray pipe cleaners into quarters, which should produce a total of 100 pieces that are 3 inches long. Then, cut the green pipe cleaners in the quantities and lengths specified in the video, taking care to minimize waste. This approximates a normal distribution of lengths and should result in an additional 100 pieces total. NOTE: One 12-inch pipe cleaner should yield multiple lengths, for example, a 5-inch piece and a 7-inch...
FAQs
The strength of the T-cell receptor signal is decisive. Thymocytes receiving weak to moderate signals through recognition of self-peptide–MHC complexes are retained, whereas cells that fail to bind MHC undergo apoptosis. This signal-dependent screening reduces nonfunctional thymocyte populations and ensures that surviving cells can participate in MHC-restricted adaptive immunity.
Cortical thymic epithelial cells provide the self-peptide–MHC complexes that developing thymocytes must recognize. Their location in the thymic cortex places this interaction within the developmental setting where receptor function is tested. Consequently, they help determine which immature T cells survive and establish the MHC recognition requirement needed for later immune activity.
Recognition of MHC class II directs developing thymocytes toward the CD4 lineage, while recognition of MHC class I directs them toward the CD8 lineage. Thus, the selection interaction does more than preserve viable cells: it links the type of MHC recognized by the receptor to the functional identity of the emerging T cell.
Researchers can examine survival during thymic development, recognition of self-MHC, and the resulting distribution of CD4 and CD8 cells. These outcomes reveal whether developing T-cell populations have acquired MHC restriction and whether nonfunctional cells have been removed. Such analyses provide a framework for investigating how functional adaptive immunity is established.
Positive Selection creates the developmental foundation for studying T-cell responses to infection. Because it preserves cells capable of recognizing self-MHC, it determines whether a functional, MHC-restricted T-cell population is available for later adaptive immune activity. Investigators can therefore connect developmental outcomes in the thymus with the capacity to analyze infection responses.
Altered survival, MHC recognition, or CD4 and CD8 lineage formation can provide context for studying immunodeficiency and disrupted immune development. The process also connects thymic screening with tolerance research because developing cells are evaluated against self-peptide–MHC complexes. These outcomes help investigators identify which stage of T-cell development may be affected.