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 V H and V L (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 tekutinyFunkction IgG (subclass IgG 1-4 ) -monomer serum, intersticial liquid opsonization (the main Ig) neutralization complement activation (classic.) passing through the placenta (the only Ig) secondary immune response (the main Ig) IgA - serum - mucose (dimer) 0,9-3,5 6serum, seromucinous secretions defense of mucosa (neutralizace) opsonization IgM - pentamer - monomer (BCR) 0,9-2,56serum, membrane of B lymfo complement activation (classic.) primary immune response (the first and main Ig) produced by fetus during IU infection receptor for Ag (BCR) IgD -monomer 0,13serum, membrane of B lymfo receptor for Ag (BCR) IgE -monomer 3x serum, interst.liquid mastocyte and basophils surface anti-helminth defense immediate 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 (C H ); 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/ postnatal B lymfo: mainly IgM production immediately after the birth (adult conc. between the year ) 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

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)