2. Tolerance:
“Tolerance is specific immunologic unresponsiveness
(i.e., an immune response to a certain antigen [or
epitope] does not occur, although the immune system is
otherwise functioning normally).”
In general, antigens that are present during embryonic
life are considered “self ” and do not stimulate an
immunologic response (i.e., we are tolerant to those
antigens).
The lack of an immune response in the fetus is caused
by the deletion of self-reactive T-cell precursors in the
thymus.
3. Types of Tolerance:
T Cell Tolerance
Central tolerance (inside the thymus)
Peripheral tolerance (outside the thymus)
B Cell Tolerance
Note: Although both B cells and T cells
participate in tolerance, it is T-cell tolerance
that plays the primary role.
4. Types of Tolerance:
Tolerance to “self” acquired within the thymus is
called central tolerance.
Whereas, the tolerance acquired outside the
thymus is called peripheral tolerance. ( mostly in
spleen, lymph nodes)
Peripheral tolerance is necessary because some
antigens are not expressed in the thymus and
therefore some self reactive T cells are not killed
in the thymus.
6. Clonal Deletion:
T lymphocytes acquires the ability to distinguish
“self” from “non-self” in the fetal thymus by the
process called clonal deletion.
It is the process that involves the killing of T cells
(“negative selection”) that react against antigens
present in the fetus at that time.
The self reactive cells die by a process of
programmed cell death called apoptosis.
For negative selection and clonal deletion to be
efficient, the thymic epithelial cells must display a
vast repertoire of “self ” proteins.
8. Clonal Deletion:
A transcriptional regulator called the autoimmune
regulator (AIRE) enhances the synthesis of this array of
self proteins.
The AIRE transcription factor also functions in the
peripheral lymphoid organs such as the spleen and
lymph nodes, where it contributes to peripheral
tolerance.
Mutations in the gene encoding the AIRE protein result
in the development of an autoimmune disease called
autoimmune polyendocrinopathy.
9. Polyendocrinopathy:
APS type 1, also known as APECED (autoimmune
polyendocrinopathy-candidiasis-ectodermal dystrophy).
MEDAC (multiple endocrine deficiency autoimmune
candidiasis syndrome).
Juvenile autoimmune polyendocrinopathy.
Whitaker's syndrome.
12. Clonal Anergy:
“Clonal anergy is the term used to describe self-
reactive T cells that are not activated because proper
co-stimulation does not occurs.”
Mechanism of clonal anergy:
Inappropriate presentation of antigen, leading to a
failure of interleukin-2 (IL-2) production.
Inappropriate presentation is due to a failure of
“co-stimulatory signals”
For instance, sufficient amounts of IL-1 might not
be made, or cell surface proteins, such as CD28 on
the T cell and B7 on the B cell, might not interact
properly, leading to a failure of signal transduction
by RAS proteins.
15. Clonal Anergy:
For example, the inhibitory protein CTLA-4 on the surface
of the T cells may displace CD28 and interact with B7,
resulting in a failure of T-cell activation.
16. Clonal Anergy:
• Furthermore, B7 is an inducible protein, and failure to induce
it in sufficient amounts can lead to anergy.
• In addition, the co-stimulatory proteins, CD40 on the B cell
and CD40L on the helper T cell, may fail to interact properly.
17. Clonal Anergy:
The failure of co-stimulatory signals most often occurs when there is
an insufficient inflammatory response at the site of infection.
The presence of microbes typically stimulates the production of pro-
inflammatory cytokines such as Tumor Necrosis Factor and IL-1.
If the inflammatory response is insufficient (i.e. adjuvant effect of
the cytokines is inadequate, the T cells will not be activated).
18. Clonal Ignorance:
“Clonal ignorance refers to self-reactive T cells that
ignore self antigens.”
These self-reactive T cells are either kept ignorant by
physical separation from the target antigens (e.g.,
the blood–brain barrier).
Or ignorance to self antigens may occurs if the
antigens are present in small amounts.
21. B-Cell Tolerance:
B cells also become tolerant to “self” by two mechanisms:
Clonal Deletion:
probably while the B-cell precursors are in the bone
marrow.
Clonal Anergy:
Of B cells in the periphery.
Note: However, tolerance in B cells is less complete than
in T cells, an observation supported by the finding that
most autoimmune diseases are mediated by antibodies.
24. Receptor Editing:
B cells bearing an antigen receptor for a self protein
can escape clonal deletion (apoptosis) by a process
called receptor editing.
In this process, a new, different light chain is produced
that changes the specificity of the receptor so that it no
longer recognizes a self protein.
This reduces the risk of autoimmune diseases and
increases the repertoire of B cells that can react against
foreign proteins.
It is estimated that as many as 50% of self-reactive B
cells undergo receptor editing. T cells do not undergo
receptor editing.
27. Induction Of Tolerance:
Whether an antigen will induce tolerance rather than an
immunologic response is largely determined by the following:
The immunologic maturity of the host:
Neonatal animals are immunologically immature and do
not respond well to foreign antigens.
The structure and dose of the antigen:
A very simple molecule induces tolerance more readily
than a complex one.
Very high or very low doses of antigen may result in
tolerance instead of an immune response.
Purified polysaccharides or amino acid copolymers injected
in very large doses result in “immune paralysis” —a lack of
response.
28. Induction Of Tolerance:
Other aspects of the induction or maintenance of
tolerance are as follows:
T cells become tolerant more readily and remain
tolerant longer than B cells.
Administration of a cross-reacting antigen tends to
terminate tolerance.
Administration of immunosuppressive drugs
enhances tolerance (e.g., in patients who have
received organ transplants).
Tolerance is maintained best if the antigen to which
the immune system is tolerant continues to be
present.