Immunoglobulins, immune response

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Immunoglobulins, immune response

• Martin Liška

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1. The structure of immunoglobulins
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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

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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 Fc-receptor)

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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

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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

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IgA

• found both in serum and seromucinous secretions
• defense of mucosa
• opsonization
• does not activate complement

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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

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• the best classical way complement activator
• does not bind phagocytes Fc receptor, but
  substantially enhances phagocytosis through
  complement activation
• biological half life 6 days

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IgD

• free form in serum, bound on B cells membrane
• antigen receptor on B cells

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IgE

• in normal conditions low amounts in serum
• mainly bound on mast cells (binds through FceR)
• anti-helminth defense
• immediate type allergic reactions

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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

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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

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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

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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

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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

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Mechanisms contributing to antibody diversity

• chance recombinations
• imprecise joining of V, D, J genes
• N-region additions
• extensive mutations involving variable-region
  genes after antigen exposure

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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)

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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 H-
  and one type of L-chain

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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 l-chain isotype

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Clonal expansion

• proliferation of lymphocytes activated by
  reaction with an antigen
• all lymphocytes of generated clone have the
  identical antigenic specifity

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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)

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The utilization of monoclonal antibodies

• Diagnostics (flow cytometry, ELISA,
  autoantibodies etc.)
• Treatment (anti-IgE, anti-TNF-a, anti-CD3)

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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

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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 antigen-specific 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)

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Phases of humoral immune responses
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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

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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

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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

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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)

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Ontogenesis of immune response

• a/ prenatal

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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

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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

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B lymphocytes

• Precursors from day 8
• Fetal B lymphocytes express IgM on their surface
• Synthesis of specific antibodies start at week
  20-24, but IgAM levels are in fact undetectable,
  IgG production starts after birth

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Monocytes-macrophages

• Macrophages can be detected by week 3-4
• Mature monocytes appear by month 5 in fetal
  circulation

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Neutrophils

• Mature cells are detectable from week 12-14

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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 childs 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., IgGA between the age of 10-15 yr.
• Humoral response to polysaccharide antigen arises
  by the age of 2 yr.

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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

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Innate imunity

• Newbornsphagocytes have generally decreased
  functional ability, activity of NK-cells is
  decreased
• Decreased total complement activity
  (concentration of its compounds is of 35-70 of
  adults)

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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