Immunoglobulins,
immune response
Martin Liška
1. The structure of
immunoglobulins
2. Isotypes
• (in principle) classes of antibodies distinguished on
the basis of H chain structure differences
• 5 types: m (IgM), d (IgD), g (IgG), a (IgA) and e
(IgE)
• in addition, we can distinguish subtypes of
antibodies within some classes (IgG, IgA) based
on their H chain differences
3. Domains and their
biological function
• in principle: domains of V regions form a
recognizing unit and domains of C regions
determine secondary biological functions of
antibody (i.e. biological half life,
distribution in the body, binding
complement, binding to cells through Fcreceptor)
4. Variable region of Ig molecule
• hypervariable loops are concentrated at the
spikes of variable regions where antigen
binding sites are localized
• the binding site specifity is determined by
amino acid sequences and both by
morphology and shape of the loop
5. The biological features of
distinct Ig classes
IgG
• the most abundant serum Ig
• the most important Ig of secondary immune
response
• the only Ig which passes through the placenta
• the main opsonizing Ig
• activates complement via classical pathway
• biological half life 21 day
IgA
• found both in serum and seromucinous
secretions
• defense of mucosa
• opsonization
• does not activate complement
IgM
• in pentamer form is found in serum; in
monomer form is bound on membrane of B
cells
• prevailing antibody of primary immune
response
• high-effective agglutinant and cytolytic agent
• usually isohaemagglutinins and natural
antibodies
• the best classical way complement
activator
• does not bind phagocytes Fc receptor, but
substantially enhances phagocytosis
through complement activation
• biological half life 6 days
IgD
• free form in serum, bound on B cells
membrane
• antigen receptor on B cells
IgE
• in normal conditions low amounts in serum
• mainly bound on mast cells (binds through
FceR)
• anti-helminth defense
• immediate type allergic reactions
Ig subclasses
• differences in H chain structure and
biological properties
• IgG1 a IgG3 participate in defence against
viral and bacterial protein antigens
• IgG2 ensures defence against antigens
which does not require help from T
lymphocytes
6. Allotypic and idiotypic
variations
• allotypes = allelic variants of isotypes
• idiotypes = structural determinants localized in
variable region connected with the ability of antigen
binding
• idiotopes = unique set of antigenic determinants
(epitopes) of variable portion of an antibody
(idiotype is the sum of idiotopes)
• anti-idiotypic antibodies = directed against
idiotypes, in principle reflect an antigen
7. Genetic basis of Ig production
a/ L chains genes
K chain – genes located on chromosome 2
- V, J and C segments
 chain – encoded in similar complex of
genes on chromosome 22
b/ genes encoding H chain
• more complicated
• localized on chromosome 14
• V, D, J, C segments (genes encoding
individual segments contain more regions
compared with L chains)
• during completion of V/D/J exon, gene
rearrangement occurs
Development of B lymphocytes
• Lymphoid progenitor → pro-B cells
• During maturation from pro-B cells into pre-B cells:
Ig genes of the heavy chain recombine; pre-B cells
express pre-BCR
• During maturation from pre-B cells into B cells: Ig
genes of the light chain recombine
• Immature B cells express membrane IgM
• Mature B cells express membrane IgM and IgD =
BCR and are able to respond to antigen in peripheral
lymphoid tissues
Mechanisms contributing to antibody
diversity:
•
•
•
•
chance recombinations
imprecise joining of V, D, J genes
N-region additions
extensive mutations involving variableregion genes after antigen exposure
Isotype switching
• during the immune response, plasma cells
switch from producing IgM to IgG or to
another Ig class (IgA, IgE)
• the switch involves a change in the H-chain
constant domains (CH)
• no changes in antigen-binding specifity !
(no alteration in the L chain or in the
variable portion of H chain)
Allelic exclusion
• once the process of rearrangement on one of
chromosomes is successful, then all attempts on
second chromosome are stopped
• the same rule governs both for H- and L-chains
• every single B cell produces only one type of Hand one type of L-chain
Clonal restriction
• each B cell expresses identical copies of an
antibody that is specific for single epitope
• when a B cell divides, the chromosomes in its
progeny cells bear the selected allelic genes, and
these genes do not undergo any further V/J or
V/D/J rearrangements
• immunoglobulins produced by given B cell and its
progeny are identical in epitope specifity and in
k- or -chain isotype
Clonal expansion
• proliferation of lymphocytes activated by
reaction with an antigen
• all lymphocytes of generated clone have the
identical antigenic specifity
Monoclonal antibodies
• immunoglobulins arising from a single
clone of B cells, or more precisely cells
artificially created by hybridisation of B
lymphocytes of specific antigenic specifity
(= produced Ig have the same antigenic
specifity) with tumor cell (= cells are
„immortal“)
The utilization of monoclonal
antibodies:
• Diagnostics (flow cytometry, ELISA,
autoantibodies etc.)
• Treatment (anti-IgE, anti-TNF-a, anti-CD3)
Humoral immune response
• The recognition of antigen by specific Ig on the
surface of naive B lymphocyte
• The binding of antigen cross-links Ig receptors of
specific B cells and then activation signals are
delivered inside the B cell; the necessary second
signal is provided by a breakdown product of the
complement protein C3
• Clonal expansion of B cell and secretion of low
levels of IgM
Humoral immune response
• Protein antigens activate antigen-specific T helper cells which
stimulate B cell; antigen presentation of these antigens to T
helper cells is required
• T helper cells exprime CD40L on their surface and secrete
cytokines → proliferation and differentiation of antigenspecific B cells, isotype switching
• Affinity maturation = affinity of antibodies for protein
antigens increases with prolonged or repeated exposure to the
antigens (B cells migrate into follicles and form germinal
centers → proliferate rapidly and their Ig V genes undergo
extensive somatic mutations; at the same time, the antigen
complexed with secreted antibody is displayed by FDC → B
cells that recognize the antigen with high affinity are selected
to survive)
Phases of humoral immune
responses
Primary immune response
• First antigen exposure
• The amounts of antibody produced is smaller
2 types of antigens:
• T-dependent – help from antigen-specific T
helper cells is required; protein antigens
• T-independent – antibody production is
induced directly, without the involvement of
T helper cells; typically polysaccharides,
lipids
Secondary immune response
• Subsequent antigen exposure
• Higher amount of antibodies is produced
• With protein antigens, secondary responses
show increased isotype switching and
affinity maturation (= production of
antibodies with increased affinity to
antigen)
• Memory cells involvement
Affinity and avidity of antibodies
• affinity = the strength of the binding
between a single binding site of a molecule
(e.g.antibody) and a ligand
• avidity = expresses the strength of
interaction of polyvalent antibody with a
polyvalent antigen
Effector functions of antibodies
• Neutralization of microbes and their toxins
• Opsonization of microbes (binding to phagocytes
through Fc-receptors, stimulation of their
microbicidal activity)
• ADCC (Antibody-dependent cell-mediated
cytotoxicity) – microbe, which is opsonized by
IgG, is killed by NK-cell after binding of
immunocomplex to Fc-receptor
• Complement system activation (classical
pathway)
Ontogenesis of immune
response
a/ prenatal
Hematopoiesis
• Mesoblast – from 2nd (3rd) week of
gestation
• Liver – from 6th (8th) week of gestation, in
liver hematopoiesis persists whole prenatal
period
• Bone marrow – from 10th (12th) week of
g., from 20th week the main organ of
hematopoiesis
T lymphocytes
• Precursors from week 7, from week 8-9
lymphocytes move into thyme, where they
differentiate
• TCR gene segments rearrangement,
expression of TCR on the surface of T
lymphocytes
• Selection
B lymphocytes
• Precursors from day 8
• Fetal B lymphocytes express IgM on their
surface
• Synthesis of specific antibodies start at
week 20-24, but IgA+M levels are in fact
undetectable, IgG production starts after
birth
Monocytes-macrophages
• Macrophages can be detected by week 3-4
• Mature monocytes appear by month 5 in
fetal circulation
Neutrophils
• Mature cells are detectable from week 1214
Postnatal
B lymphocytes
• relative counts decrease after the birth
• respond to immunization presumably by IgM
production, switching to other isotypes is slower
• slow increase of child’s own IgG connected with
decrease of maternal IgG levels (by month 3-6)
• IgM reaches levels common i adults at the age of
1-3 yr., IgG+A between the age of 10-15 yr.
• Humoral response to polysaccharide antigen arises
by the age of 2 yr.
T lymphocytes
• More than 90% are naive, but their numbers
decrease in adult age
• Proliferation under mitogen stimulation
similar to adults X response to specific
antigens only after contact with them
• Lower cytotoxic activity of T lymphocytes
Innate imunity
• Newborns´phagocytes have generally
decreased functional ability, activity of NKcells is decreased
• Decreased total complement activity
(concentration of its compounds is of 3570% of adults)
c/ Old age
• decreased cytotoxicity of NK-cells and
macrophages
• decreased resistance against viral infections,
decreased anti-tumour immunity
• switching from Th1 to Th2
• weaker humoral response under new stimuli
• increased production of autoantibodies