Clonal selection

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Clonal selection theory
)
For the use of clonal selection in general horticulture, see
Clonal (plant). For the use of clonal selection in viticulture, see Propagation of grapevines
.

Clonal selection theory of lymphocytes:
1) A hematopoietic stem cell undergoes differentiation and genetic rearrangement to produce
2) immature lymphocytes with many different antigen receptors. Those that bind to
3) antigens from the body's own tissues are destroyed, while the rest mature into
4) inactive lymphocytes. Most of these never encounter a matching
5) foreign antigen, but those that do are activated and produce
6) many clones of themselves.

In

antigens.[3]

The theory states that in a pre-existing group of lymphocytes (both B and T cells), a specific antigen activates (i.e. selects) only its counter-specific cell, which then induces that particular cell to multiply, producing identical

adaptive immunity.[7]

Postulates

The clonal selection theory can be summarised with the following four tenets:

Early work

In 1900, Paul Ehrlich proposed the so-called "side chain theory" of antibody production. According to it, certain cells exhibit on their surface different "side chains" (i.e. membrane-bound antibodies) able to react with different antigens. When an antigen is present, it binds to a matching side chain. Then the cell stops producing all other side chains and starts intensive synthesis and secretion of the antigen-binding side chain as a soluble antibody. Though distinct from clonal selection, Ehrlich's idea was a selection theory far more accurate than the instructive theories that dominated immunology in the next decades.

In 1955, Danish immunologist

serum prior to any infection. The entrance of an antigen into the body results in the selection of only one type of antibody to match it. This supposedly occurred by certain cells phagocytosing the immune complexes and somehow replicating the antibody structure to produce more of it.[8]

In 1957,

David W. Talmage
hypothesized that antigens bind to antibodies on the surface of antibody-producing cells and "only those cells are selected for multiplication whose synthesized product has affinity for the antigen". The key difference from Ehrlich's theory was that every cell was presumed to synthesize only one sort of antibody. After antigen binding the cell proliferates, forming clones with identical antibodies.

Burnet's clonal selection theory

Later in 1957, Australian immunologist

antigenic determinants that occur in biological material other than those characteristic of the body itself. Each type of pattern is a specific product of a clone of lymphocytes and it is the essence of the hypothesis that each cell automatically has available on its surface representative reactive sites equivalent to those of the globulin they produce. When an antigen enters the blood or tissue fluids it is assumed that it will attach to the surface of any lymphocyte carrying reactive sites that correspond to one of its antigenic determinants. Then the cell is activated and undergoes proliferation to produce a variety of descendants. In this way, preferential proliferation is initiated of all those clones whose reactive sites correspond to the antigenic determinants on the antigens present in the body. The descendants are capable of active liberation of soluble antibody and lymphocytes, the same functions as the parental forms.[5][9]

In 1958, Gustav Nossal and Joshua Lederberg showed that one B cell always produces only one antibody, which was the first direct evidence supporting the clonal selection theory.[6]

Theories supported by clonal selection

Burnet and

immunological tolerance
, a phenomenon also explained by clonal selection. This is the organism's ability to tolerate the introduction of cells prior to the development of an immune response as long as it occurs early in the organism's development. There are a vast number of lymphocytes occurring in the immune system, ranging from cells that tolerate self tissue to cells that do not. However, only cells tolerant of self tissue survive the embryonic stage. If non-self tissue is introduced, lymphocytes that develop are the ones that include the non-self tissues as self tissue.

In 1959, Burnet proposed that under certain circumstances, tissues could be successfully transplanted into foreign recipients. This work has led to a much greater understanding of the immune system and also great advances in tissue transplantation. Burnet and Medawar shared the Nobel Prize in Physiology or Medicine in 1960.

In 1974, Niels Kaj Jerne proposed that the immune system functions as a network that is regulated via interactions between the variable parts of lymphocytes and their secreted molecules. Immune network theory is firmly based on the concept of clonal selection. Jerne won the Nobel Prize in Physiology or Medicine in 1984, largely for his contributions to immune network theory.

See also

References

  1. S2CID 40609269
    .
  2. S2CID 24741671
    .
  3. S2CID 4279640
    .
  4. ISBN 9780815342434
    .
  5. ^
    S2CID 19122290
    .
  6. ^
    PMID 13552693
    .
  7. PMID 24141853
    .
  8. S2CID 40609269
    .
  9. S2CID 29935594
    .

Further reading

External links
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