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clonal selection theory 179 Clostridium immunity
Antigen
B cells
Production/
maturation
clonal selection theory
A selective theory of antibody formation proposed by
F.M. Burnett, who postulated the presence of numerous
antibody-forming cells, each capable of synthesizing its
own predetermined antibody. One of the cells, after having
been selected by the best-fitting antigen, multiplies
and forms a clone of cells that continue to synthesize the
same antibody. Considering the existence of many different
cells, each capable of synthesizing an antibody of a
different specificity, all known facts of antibody formation
are easily accounted for. An important element of the
clonal selection theory was the hypothesis that many cells
with different antibody specificities arise through random
somatic mutations during a period of hypermutability
early in life. Also early in life, the “forbidden” clones of
antibody-forming cells (i.e., the cells that make antibody
to the animal’s own antigen) are still destroyed after
encountering these autoantigens. This process accounts for
an animal’s tolerance of its own antigens. Antigen would
have no effect on most lymphoid cells, but it would selectively
stimulate those cells already synthesizing the corresponding
antibody at a low rate. The cell surface antibody
would serve as receptor for antigen and proliferate into
a clone of cells, producing antibody of that specificity.
Burnett introduced the forbidden clone concept to explain
autoimmunity. Cells capable of forming antibody against a
normal self antigen were “forbidden” and eliminated during
embryonic life. During fetal development, clones that
react with self antigens are destroyed or suppressed. The
subsequent activation of suppressed clones reactive with
self antigens in later life may induce autoimmune disease.
D.W. Talmage proposed a cell selection theory of antibody
formation that was the basis for Burnett’s clonal selection
theory.
clone
A cell or organism that develops from a single progenitor
cell and has exactly the same genotype and phenotype of
the parent cell. Malignant proliferation of a clone of plasma
cells in multiple myeloma represents a type of monoclonal
gammopathy. The fusion of an antibody-producing B cell
with a mutant myeloma cell in vitro by the action of polyethylene
glycol to form a hybridoma that is immortal and
Clonal selection theory.
Memory cells
Production
of AB3
produces monoclonal antibody is an example of the in vitro
production of a clone.
cloned DNA
A DNA fragment or gene introduced into a vector and replicated
in eukaryotic cells or bacteria.
cloned enzyme donor immunoassay
A homogeneous enzyme immunoassay (EIA) based on
the modulation of enzyme activity by bound fragments
of β-galactosidase.
cloned T cell line
A lineage of T lymphocytes that grow continuously from a
single progenitor cell. Stimulation with antigen from time to
time is necessary to maintain their growth. They are used
in experimental immunology to investigate T cell function
and specificity.
clonotypic
An adjective that defines the features of a specific B cell
population’s receptors for antigen that are products of a single
B lymphocyte clone. Following release from the B cells,
these antibodies should be very specific for antigen, have
a restricted spectrotype, and possess at least one unique
private idiotypic determinant. Clonotypic may also describe
the features of a particular clone of T lymphocytes’ specific
receptor for antigen with respect to idiotypic determinants,
specificity for antigen, and receptor similarity from one
daughter cell of the clone to another.
Clostridium immunity
Clostridia produce disease by releasing exotoxins. They
may produce more than one toxin, and each one is immunologically
unique. For example, each of the five types
of Clostridium perfringens produces a different toxin.
Clostridia enter the host by many routes to produce disease.
C. perfringens gains access through traumatic or surgical
wounds to produce gas gangrene and wound infections.
The microorganism is aided by a poor blood supply in the
area of the wound. Clostridia divide and produce toxins
that cause disease. When antibiotics upset the normal bowel
flora, C. difficile may multiply and induce colitis. The toxins
released from this organism act on intestinal epithelial
cells and produce diarrhea and chronic inflammation. C.
botulinum does not grow in the host but forms toxins in
contaminated food that when ingested leads to disease.
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