Positive selection

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 B-Cell Differentiation

  • B Cells develop in way that is similar but different to T cells, and undergo clonal selection
  • This is a form of positive selection.
  • They are also tested for autoreactivity through negative selection.
  • Clonal selection is theory in which it is postulated that a B cell expresses antigen specific receptors before antibodies even encounter an antigen or before that antigen is ever found in the body.
  • Following the initial infection, random mutations during clonal selection could have produced memory B cells that can more easily bind to antigens than the original B cells could.

• Lymphocytes

  • They have an immunosuppressive effect that inhibits cell-mediated immunity at the end of a response and destroys autoimmune T cells that aren't filtered out by negative selection in the thymus.
  • While in the bone marrow, B cells are sorted through positive and negative selection in a manner somewhat similiar to T cell maturation in the thymus, with the same process of killing B cells that are nonreactive to antigens or reactive to self-antigens.

• Antimicrobial Proteins

  • These peptides have a stronger positive than negative charge, which is an important component of their selectivity.
  • The amphipathicity (hydrophilic and hydrophobic ends) and positive charge of peptides are their defining structural features.
  • The initial contact between the peptide and target organism is electrostatic due to the force of negative and positive ionic charge.
  • The peptides are selective and thus more likely to adhere to bacterial cell membranes than to cell membranes of the host cells.
  • The peptides have a greater positive charge than negative charge, while bacterial cell membranes have a greater negative charge than host cell membranes.

• Erythroblastosis Fetalis (Hemolytic Disease of the Newborn)

  • In terms of natural selection, she was selected against.
  • In other words, if a mother has anti-RhD (D being the major Rhesus antigen) IgG antibodies as a result of previously carrying a RhD-positive fetus, this antibody will only affect a fetus with RhD-positive blood.
  • A drawback to pre-partum administration of RhIG is that it causes a positive antibody screen when the mother is tested, which can be difficult to distinguish from natural immunonological responses that result in antibody production.
  • If agglutination occurs, the indirect Coombs test is positive.
  • If this produces agglutination of RBCs, the direct Coombs test is positive, a visual indication that antibodies (and/or complement proteins) are bound to the surface of red blood cells.

• Lipids: Sources, Uses in the Body, and Dietary Requirements

  • Both omega-6 and omega-3 are 18-carbon polyunsaturated fatty acids that differ in the number and position of their double bonds.
  • Alpha-linolenic acid is found in the green leaves of plants, selected seeds, nuts, and legumes, and particularly in flax, rapeseed, walnut, and soy.
  • Numerous studies have shown that the consumption of omega-3 fatty acids has positive benefits in terms of infant development, cancer, cardiovascular disease, and mental illnesses such as depression, attention-deficit hyperactivity disorder, and dementia.

• Clonal Selection and T-Cell Differentiation

  • Antigens are "selected" to form clones of themselves, both memory and effector.
  • Clonal selection is an theory that attempts to explain why lymphocytes are able to respond to so many different types of antigens.
  • Clonal selection assumes that lymphocytes already have receptors for that antigen, and are selected during antigen presentation because they already have that unique antigen receptor.
  • Clonal selection means that an antigen is presented to many circulating naive B and (via MHC) T cells, and that the lymphocytes that match the antigen are "selected" to form clones of themselves, both memory and effector.
  • Clonal selection may also be used during negative selection during T cell maturation.

• Anatomical Position

  • When an organism is in its standard anatomical position, positional descriptive terms are used to indicate regions and features.
  • All descriptions refer to the organism in its standard anatomical position, even when the organism's appendages are in another position.
  • The standard anatomical position is agreed upon by the international medical community.
  • The basis for the standard anatomical position in humans comes from the supine position used for examining human cadavers during autopsies.
  • The regions of the body in standard anatomical position, in which the body is erect.

• Taste and Smell at Birth and in Old Age

  • Intrauterine olfactory learning may be demonstrated by behavioral evidence that newborn infants respond positively to the smell of their own amniotic fluid.
  • As a result, it seems natural selection should favor the development of a means to help maintain and establish effective breast feeding.
  • These breast odors bring forth positive responses in neonates from as young as one hour or less through several weeks postpartum.
  • A significant change takes place in the regulation of olfaction just after birth so that odors related with the offspring are no longer aversive, allowing the female to positively respond to her babies.

• Cerebral Lobes

  • The frontal lobe is an area in the mammalian brain located at the front of each cerebral hemisphere and positioned anterior to (in front of) the parietal lobe and superior and anterior to the temporal lobes.
  • Dopamine tends to limit and select sensory information that the thalamus sends to the forebrain.
  • The parietal lobe is a part of the brain positioned above (superior to) the occipital lobe and behind (posterior to) the frontal lobe.
  • This enables regions of the parietal cortex to map objects perceived visually into body coordinate positions.