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In: Biology

Explain clonal selection and how it works to protect you against many different infectious agents.

Explain clonal selection and how it works to protect you against many different infectious agents.

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Ans.

Clonal selection ;

  • It explain the process by which a single T or B cell, that recognizes a foreign antigen in the body, is chosen from the already available cell pool of various different antigen specificities.
  • This single chosen T or B cell then reproduce to generate a population of clonal cell capable of eliminating the antigen.
  • Therefore, according to the theory of clonal selection, an individual lymphocyte expresses a receptor that is unique for an antigen, recognized even before the encounter of antibodies with that antigen.
  • Upon binding of an antigen to a cell, the cell becomes activated and results in the proliferation of clone daughter cells.
  • This theory of clonal selection has been widely accepted as a model of how response to an infection is shown by the immune system and how several specific B and T cell types are being selected against specific foreign antigens invading the body.

The clonal selection theory postulates 4 predictions which are ;

  1. Only a single type of receptor with a unique specificity is found on each lymphocytes.
  2. For the activation of cell, receptor activation is required.
  3. Receptor specificity of the differentiated effector cells will be identical to the parental cells.
  4. Those lymphocytes which have receptor for self molecules will be purged at an early stage.
  • This theory was first given by the Frank Macfarlane Burnet, an Australian immunologist, who described immunological memory as the cloning of two types of lymphocytes.
  • According to him, one of the clones immediately act to combat the infection whereas the other one is a longer lasting clone that remain in the immune system for a long period of time and provides immunity to that antigen.

Steps of clonal selection of lymphocytes can be laid out as follows ;

  1. A hematopoietic stem cell undergoes the stage of differentiation and genetic rearrangement.
  2. This results in the production of immature lymphocytes which have many different types of antigen receptors.
  3. Lymphocytes that binds to the antigen from body's own tissues are deleted and the remaining lymphocytes get mature into the dormant lymphocytes.
  4. Those lymphocytes that encounter an antigen, are activated and generate many of their clones.
  • Thus, according to this theory, B cells generally exist as clones and all of the B cells are derived from the same progenitor cell and can bind to the same epitope of an antigen.
  • There may be some important consequences of such kind of clonality because immunogenic memories relies on it.
  • Upon encounter of a specific antigen, an individual B cell or it's clone divides to generate more B cells.
  • Majority of these B cells differentiate into plasma cells that release antibodies into blood, capable of binding to epitope identical to that responsible for proliferation in the earlier stage.
  • A small part of these cells survive as memory cells that can recognize identical epitope only and number of memory cells increases with each cycle.
  • This maturation is associated with affinity maturation responsible for inducing the survival of B cells that bind with high affinity to the particular antigen.
  • This subsequent amplification is called as secondary immune response which posses improved immune respnse specificity.
  • Dormant B cells are called as naive lymphocytes while those that have encountered an antigen, become activated and differentiate into fully functional effector B lymphocytes.

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