Immunoglobulins - structure.Immunoglobulins - function.Genetic background of immunoglobulin production.Biological and chemical characteristics of immunoglobulin classes IgG and IgA.Biological and chemical characteristics of immunoglobulin classes IgM, IgD and IgE.Isotype switching. Idiotypes and anti-idiotypes - their role. Immunological memory.Ontogenesis of the immune response.Primary immune response.Secondary immune response .Effector functions of immunoglobulins.

Simona Kaftanová

The structure of immunoglobulins• 4 polypeptid chains:

2 identical heavy chains (H)2 identical light chains (L)

H chains: μ, δ, γ, α, ε L chains: κ, λ

Izotypys = classes of antibodies :IgM (μ)IgD (δ)IgG (γ)IgA (α)IgE (ε)

Ig fragments produced by proteolytic digestion (papain):• Fab fragments (portions) contain the antigen binding

sites of the antibody• Fc fragment binds to cells through Fc-receptor

Domains:• domains of V regions form a recognizing unit for Ag• domains of C regions determine secondary biological

functions of antibody

The structure of immunoglobulins

Paratop - the binding site for Ag (for it’s epitop )= part of Ig made of hypervariable regions of VH and VL (hypervariable region – spike of variable region with hypervariable loops of amino acid (AA) sequences; the binding site specificity is determined by AA sequences and both by morphology and shape of the loop)

Idiotop – individual and rarely structures localized in the variable region; some of them are identical with the binding site, some lie except of the paratop

Idiotyp - the sum of idiotopes of an Ig molecule

The structure of immunoglobulins

Anti-idiotypic antibodies• antibodies of the 2nd generation (idiotyp of the 1st gen. of antibodies looks like an antigen, idiotyp can start production of Abb against itself – antiidiotypic antibodies)• in principle reflect the antigen • other new antibodies (3rd generation) are produced against them → antiantiidiotypic antibodies → idiotypic net• regulatory function

idiotyp of the 1st generat. of antibodies

epitops

antibodies of the 2nd generation

antibodies of the 3rd generation

Function of Igg

– antibodies – solubil Igg – antigen receptor (BCR) – (IgM a IgD on the surface of B cells)

• differences in H chain structure (Fc ftagment) determine secondary

biological function of antibodies: biological half life, distribution in the body, passing through the placenta, complement activation, binding to cells through Fc-receptor – opsonization, IgE binding on mastocytes etc.

Izotyp Serum conc.(g/l)

Biolog. half life (days)

Tělesné tekutiny Funkction

IgG(subclass IgG1-4 )-monomer

8-18 21 serum, intersticial liquid

opsonization (the main Ig)neutralizationcomplement activation (classic.)passing through the placenta (the only Ig)secondary immune response (the main Ig)

IgA- serum - mucose (dimer)

0,9-3,5 6 serum, seromucinous secretions

defense of mucosa (neutralizace)opsonization

IgM- pentamer- monomer (BCR)

0,9-2,5 6 serum, membrane of B lymfo

complement activation (classic.)primary immune response (the first and main Ig)produced by fetus during IU infectionreceptor for Ag (BCR)

IgD-monomer

0,1 3 serum, membrane of B lymfo

receptor for Ag (BCR)

IgE-monomer

3x10-4 2 serum, interst.liquidmastocyte and basophils surface

anti-helminth defenseimmediate type allergic reactions

Genetic basis of Ig production

H chains genes (chromosome 14) • genes are structured in V, D, J, C segments (segments contain

more regions compared with L chains)– V („variability“, several hundred)– D (cca 50)– J (9)– C (encoding izotypes - Cμ, Cγ, Cα, Cδ, Cε)

L chains genes (κ – chromosome 2, λ – chromosome 22)• less complicated• V, J, C segments

a) D-J rearrangement - accidental D-J joining

b) V-D rearrangement – V-D-J joining

c) transcription if V-D-J product is readable, then splicing (V-D-J-C joining) d) translation in protein (H chain)

H chain recombination:

L chain recombination :

V-J rearrangement

transcription

splicing –VJC rearrangement

Mechanisms contributing to antibody diversity:

• chance recombinations

• imprecise joining of V, D, J genes

• extensive mutations involving variable-region 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)

• mechanism: segment Cμ is excluded and replaced by other next segment f.e. Cγ (IgG production)

• regulation – mainly IL-4

• change only in the H-chain constant domains (CH); no change in antigen-binding specificity

• 2 periods: primary, secondary • characteristic: memory cells, production antibodies with the increasing

affinity to Ag

Primary period of primary immune response:• the first Ag exposure (secondary lymf. org.) → …see Th2 based immune

reaction… • clonal expansion of B cell with identical or similar specifity to Ag →

diferenciation to plasmocytes and memory cells• some of plasmocytes back to circulation (bone marrow,...)• in 3-4 days secretion plenty of IgM with low affinity to Ag• imunokomplexes are displayed by FDC (folicular dendritic cells in

secondary lymf. org.) Exception: T-independent Ag– antibody production is induced directly,

without the involvement of T helper cells; typically polysaccharides, lipids

Primary immune response (reaction against primary infection):

Humoral immune response

a) affinity maturation : the antigen displayed by FDC is recognized by B cells → other proliferation a diferenciation of B lymfo → their Ig V genes undergo extensive somatic mutations → changes of Ig binding sites (hypervariability parts) → competition about lower amount of Ag → B cells that recognize the antigen with the highest affinity are selected to survive

b) isotype switching: start of production other Ig which is

on (mainly IgG, others IgA, IgE, IgD)) After primary immune response Abb circulate in organism for a

certain time; they stop reinfection for a certain time.

Secondary period of primary immune response :

Secondary immune response (reaction against repeatedly infection):

• activation of memory cells• increasing of the affinity maturation (production of antibodies with

increased affinity to Ag)• continuation of isotype switching (higher amount of antibodies is

produced; start of Ig production is faster (faster and higher amount of IgG)

• suppression of infection is quicker and more effectively

Effector functions of immunoglobulins

• neutralization – Igg bind a blocade critical epitops of toxins, virus and other microorganisms

• inhibition of patogen adherence – IgA blocade adherence of microorganisms on the mucose surface

• opsonization – Ig is binded on Ag; fagocytes are more able to swallow up Ag (FcR on the fagocytes surface)

• ADCC (antibody-dependent cell-mediated cytotoxicity) – IgG is binded on Ag; FcR for IgG on the NK cells surface; binding IgG and FcR is a stimul for NK degranulation on Ag (cytotoxic products)

• complement activation via classical pathway → products of

complement...

Ontogenesis of immune response

a/ prenatal• Hematopoesis – extraembryonaly (start in the week 2-3 of

gestation) → then in liver (whole prenatal period) → postnataly bone marrow (the main organ of hematopoesis)

• T-cells - precursors to thymus → diferenciation in lymfoid cells, selection.

• B-cells – synthesis interauter. IgM unable to detection (detectable concentration IgM = information about IU infection) x IgG production starts after the birth

• monocytes-makrophages – mature already in foetus• neutrophils – smaller amount

b/ postnatalB lymfo:• mainly IgM production immediately after the birth (adult conc.

between the year 1.- 3.)• IgG production increases slow (and decrease of mothernal IgG) →

the most decrease between the month 3. – 6.• humoral response to polysacharide antigen (f.e. Haemophilus)

arises by the age of 2 yr. T lymfo :• lower activity after the birth

Innate imunity:• lower ability of the function

Ontogenesis of immune response

Ontogenesis of immune response

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 but increased production of autoantibodies)