Imunoglobulins – structure and function
Production of immunoglobulins
Genetic determination of immunoglobulin production
Clonal selection theory
Antigen and epitope
Hinge region
Immunoglobulin domains – are
associated with various functions of
Protein domain
• is a region of a protein's polypeptide
chain that is self-stabilizing and
that folds independently from the rest.
Each domain forms a compact
folded three-dimensional globular
structure. Usulally held together by a
disulfidic bond.
IgG
• Proteolytic cleavage (by pepsin or papain)
results in formation of two fragments of Ig
molecule:
• Fab (antigen binding) associated mainly
with antigen specificity
• Fc (crystallizable) – associated with various
functions of immunoglobulin molecule
Molecule of IgG
Hypervaribale region of immunoglobulin
molecule binds epitope of the antigen
Variable region of immunoglobulin molecule
Hypervariable regions of immunoglobulin molecule
Clonal selection theory
Effector
cells
antigen
Memory
cells
Elimination of
autoreactive
clones
Blood and
periperal
lymphatic organs
expansion
death
death
Clonal selection theory
F.M. Burnet, 1957
• During (mainly fetal) development immunocompetent cells of
the immune system develop. Each cell is characterized by its
own antigen specific receptor. Each cell reacts only with one
concrete specific antigen.
• After exposure to autoantigen during fetal life autoreactive
clones are eliminated ( „forbidden clones“).
• If a concrete cell recognizes its specific antigen, it is stimulated,
proliferates and forms a clone = clonal selection.
• After repeated divisions the cells become terminally
differentiated cells, that does not proliferate and after some time
die.
• The cells of the clone that do not differentiate into the terminal
stage become a memory cells which will quickly react after the
second exposure to the antigen.
From the history of immunology
• 1957: Clonal selection theory: 1957
• 1961: Discovery o the thymus as an organ involved in
the immune systém reaction
• 1965 : T and B- lymphocytes determined
• 1969 discovery of the exact function of the thymus,
dichotomy of the immune system
• 1975 Positive and negative selection during the
thymocytes´ development
• 1978-1980 Organization of the immunoglobulin genes
VDJ Recombination
Germline configuration
D to J recombination
V to DJ recombination
transcription, splicing
V segments D segments J segments Constant region exons
AAA
Assembly
Adapted from Janeway 2001
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VDJ genes for BCR, and TCR
Somatic hypermutations
• The process occurs in activated B-lymphocytes, takes
place in germinal centers of secondary lymphoid organs.
• Key enzyme is AID (activation-induced deaminase).
• Mutation frequency is approx. 106 times higher than in
other parts of human genome.
• Antigen presentation by lymphoid dendritic cells to B-cells
leads to selection of clones with higher affinity – the
process is called affinity maturation.
Isotype switching
Isotype switching
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Activation and differentiation of B-lymphocytes
(clonal selection theory in B-lymphocyte development)
Primary phase of the antibody
response
• Naive or opsonised antigen captured by follicular
dendritic cells.
• Primary stimulation of B-cells in lymphoid folicles.
• The antigen also stimulates T cells (after adequate
presentation) in T-cell zones. T-cells migrate toward
the lymphoid folicles.
• Newly formed plasma cells produce ptredominatly
IgM (mainly in bone marrow).
Secondary phase of the antibody
response
• Occurs in newly formed germinal centers of
lymphoid folicles.
• Th lympocytes stimulate B-lymphocytes to
somatic hypermutations and isotype
switching.
• This leads to selection of B- cells producing
high-affinity antibodies (affinity maturation).
• Majority of B-cells producing low-affinty
antibodies die.
Development of B-cells in the bone marrow
• Stem cells: no B-cell surface markers, no rearrangement of Ig
genes.
• Pro-B lymphocyte – rearrangement of heavy chain ,
expression of several B-cell surface markers (e.g. CD19).
• Pre-B-lymphocytes VDJ of heavy chain has been completed,
m chain can be detected in cytoplasm. Pre-B receptor –
composed of m chain and surrogate chains V-preB and l5 is
expressed on the surface of the cell. Signal transduction though
this receptor is essential for B- cell development.
• Imature B-cell – light chain rearrangement (V-J) completed
B-cell receptor is composed of monomeric IgM.
• Mature B-lymphocyte has IgM and IgD B-cell receptors.
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Development of B-cells in the bone marrow
B-cell receptorHeavy chain
μ/δ
Light
chain
κ/λ
Igα Igβ
Differentiation Proliferation Survival
CD19
Pre B-cell receptor
Bruton´s tyrosine kinase (BTK)
• Key thyrosine kinase in activation, differentiation and
development of B-cells.
• Mutations of BTK lead to X-linked (Bruton´s)
agammaglobulinemia.
• BTK blockers (e.g. ibrutimib) are used for the treatment of Bcell
malignancies.
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Isotype
• The class or subclass of an immunoglobulin.
• Antigenic determinats are on constant part of
immunoglobulin molecule.
Idiotype
• An antigenic determinant on the variable
region of immunoglobulin molecule.
Interaction idiotype-antiidiotype
antigen
Antiidiotyp
idiotyp
Antibody
Ab 1
Antibody
Ab 2
Antibody
Ab 3
Paratop
Epitop
www.immunology.klimov.tom.ru/1-1.php.
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Characteristics of immunoglobulin classes
IgG
Antibody response after primary and
secondary antigen exposure
Secondary antibody responsePrimary antibody
response
IgM
IgG
Firstexporsure
Serumantibodytiters
Weeks
Secondexposuretoantigen
IgM on B-cell membrane
Expression of surface immunoglobulins on
B-cells
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Formation of Secretory IgA
• Affinity: The strength of the binding
between a single site of an antibody (one
variable region) and an epitope.
• Avidity: The overall strength of interaction
between and antibody and antigen. The
avidity depends on affinity and the valency
of interactions.
Biological half-life and serum levels of
immunoglobulin classes
• IgG: half life approx. 3-4 weeks, serum level
approx. 10 g/l.
• IgA, IgM: half life 5-6 days, serum level approx.
1-3 g/l.
• IgE: half life in plasma approx. 1 day (much more
on IgE receptors on mast cells), serum levels very
variable, several mg/l (IU/ml are used).
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Biological functions of immunoglobulin
molecules
• Activation of complement system (IgG, IgM)
• Opsonization (particularly IgG)
• Neutralization of antigens (IgG, IgA, IgM)
• Adherence interference (IgA, IgG)
• Antibody dependent cellular cytotoxicity (ADCC)
• Agglutation, precipitation (IgG, IgM)
• Mast cells degranulation (IgE)
• Transport through placenta (IgG)
• Imunoregulation (mainly IgG)
Antibody dependent cellular cytotoxicity
(ADCC)
Fc receptor
Virus-infected Cell
NK Cell
perforin granzyne
IgG
Fab
Fc
Epitope