The document summarizes Burnet's clonal selection theory of antibody production. It explains that according to this theory, lymphocyte stem cells randomly differentiate to produce mature B and T cells, each with a unique antigen receptor. When a B cell encounters the antigen it recognizes, it activates the B cell clone and causes it to proliferate and differentiate into plasma cells that secrete antibodies with the same specificity as the parental B cell receptor. The theory explains how the immune system produces antibodies and memory cells that allow for a rapid secondary immune response upon reexposure to the same antigen.

1. UNIT: I
CLONAL SELECTION THEORY

2. Synopsis
▪ Introduction
▪ Types of Immune Response
▪ B lymphocytes (b cells)
▪ Memory cells
▪ Burnet’s clonal selection theory
▪ According to clonal selection hypothesis

3. Clonal section
theroy

4. INTRODUCION
▪ Clonal selection theory is a scientific theory in
immunology that explains the functions of cells of the
immune system (lymphocytes) in response to specific
antigens invading the body.
▪ Each B cell has a specific antibody as a cell surface
receptor.
▪ The arrangement and generation of antibody genes
occurs prior to any exposure to antigen.

5. ▪ When a soluble antigen is present, it binds to the
antibody on the surface of B cells that have the
correct specificity.
▪ These B cell clones develop into antibody producing
plasma cells or memory.
▪ Only B cells, which are antigen-specific, are capable
of secreting antibodies.

7. Types of Immune Response
Primary Immune Response
This is a response to an invader the First time the invader
infects the body.
No measurable immune response for first few days.
Next 10-15 days antibody production grows steadily

8. Secondary Response
Amore rapid response to an invader the 2nd time it invades the
body.
Antibody production increases dramatically and in a much
shorter time period

9. B lymphocytes (b cells)
▪ Lymphocytes respond specifically to antigens on
foreign cells, cells infected by pathogens and toxins
released by pathogens.
▪ AN ANTIGEN IS A COMPLEX MOLECULE —
RECOGNISABLE AS SELF OR FOREIGN/ NON-
SELF
▪ ANTIGEN TRIGGERS PRODUCTION OF
ANTIBODIES
▪ ANTIBODY IS A Y-SHAPED MOLECULE WITH
A SPECIFIC RECEPTOR (BINDING SITE)

10. Memory cells
▪ Some T and B lymphocytes produced in response to
antigens by clonal selection survive long term as memory
cells.
▪ A secondary exposure to the same antigen rapidly gives rise
to a new clone of lymphocytes producing a rapid and
greater immunological response.

11. ▪ .

12. How Do B Cells Produce Antibodies?
▪ Bcells develop from stem cells in the bone marrow of
adults (liver of fetuses).
▪ After maturation B cells migrate to lymphoid organs
(lymph node or spleen).

13. Clonal Selection:
▪ When a B cell encounters an antigen it recognizes, it
is stimulated and divides into many clones called
plasma cells, which actively secrete antibodies.
▪ Each B cell produces antibodies that will recognize
only one antigenic determinant.

15. BURNET’S CLONAL SELECTION THEORY
▪ 1959, F.M. Burnet proposed the
clonal selection theory of antibody
production.
▪ The clonal selection hypothesis
states that an individual B cell
expresses receptors specific to the
distinct antigen, determined before
the antibody ever encounters the
antigen.
Sir Macfarlane Burnet

16. ▪ The clonal selection hypothesis has become
a widely accepted model for how the immune
system responds to infection and how certain
types of B and T lymphocytes are selected
for destruction of specific antigens

17. According to clonal selection
hypothesis:
▪ The lymphoid stem cells differentiate randomly to produce
different mature lymphocyte each carrying an Ag binding
receptor.
▪ Each lymphocyte bears a unique type of receptor with a
unique specificity (derived by V(D)J recombination),
determined before the antibody ever encounters the antigen

19. ▪ The occupation of a receptor by an antigen (epitope) is required
for cell activation (clonal selection), which is followed by
proliferation (clonal expansion) of this cell to form clones.
▪ The activated cells of a clone differentiate into Ab producing
cells, effector cells & memory cells.
▪ The differentiated effector cells derived from an activated
lymphocyte clone will bear receptors of identical specificity as
the parental cell.

20. ▪ The specificity of Ab produced by a lymphocyte is
identical to that of its Ag receptor.
▪ Those lymphocytes bearing receptors for self
molecules will be deleted at an early stage.

21. Steps of Clonal Selection:
▪ In the primary immune response, clonal selection produces
effector cells and memory cells that may confer life-long
immunity.
▪ In the secondary immune response, memory cells are
activated by a second exposure to the same antigen, which
initiates a rapid and anamnestic response.

22. ▪ Random somatic hypermutations during clonal
expansion cause the production of B cells with
increased antibody-binding affinity for their antigens.
▪ The clonal selection hypothesis may explain why
secondary immune responses are so effective at
preventing reinfection by the same pathogen.

24. ▪ It generally takes 4-5 days for a naive Blymphocyte that has
been activated to complete clonal expansion and differentiate
into effector B-lymphocytes.Although Burnet proposed this
theory for B cells and antibody production, but it is equally
applicable to T cells also.
▪ Clone: A clone is defined as a group of identical cells derived
from a single cell.

25. Clonal selection: When an antigen encounters the immune
system, its epitopes eventually will react only with B-
lymphocytes with B-cell receptors on their surface that more
or less fit and this activates those B-lymphocytes. This
process is known as clonal selection.

26. Reference
▪ Hodgkin, Philip D; Heath, William R; Baxter, Alan G
(2007). "The clonal selection theory: 50 years since the
revolution". Nature Immunology. 8 (10): 1019–
1026PMID 24141853.
▪ Burnet, F. M. (1976). "A Modification of Jerne's Theory of
Antibody Production using the Concept of Clonal
Selection". CA: A Cancer Journal for Clinicians. 2
▪ Murphy, Kenneth (2012). Janeway's Immunobiology 8th
Edition. New York, NY: Garland
Science. ISBN 9780815342434

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