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
;
- Only a single type of receptor with a unique specificity is
found on each lymphocytes.
- For the activation of cell, receptor activation is
required.
- Receptor specificity of the differentiated effector cells will
be identical to the parental cells.
- 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 ;
- A hematopoietic stem cell undergoes the stage of
differentiation and genetic rearrangement.
- This results in the production of immature lymphocytes which
have many different types of antigen receptors.
- Lymphocytes that binds to the antigen from body's own tissues
are deleted and the remaining lymphocytes get mature into the
dormant lymphocytes.
- 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.