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Antigens and Antibodies

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Title: Kuby Immunology 6/e Author: Kindt, Goldsby, Osborne Last modified by: sb Created Date: 12/24/2002 1:08:46 AM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Antigens and Antibodies


1
  • Chapter 3
  • Antigens and Antibodies

2
  • Hallmark molecules of adaptive immunity
  • Antibody and T-cell receptor
  • Antibody is part of the B cell receptor
  • Innate immunity recognizes patterns, whereas
    antibodies and T cell receptors have high degree
    of specificity

3
  • Antibodies and T cell receptors
  • Recognize epitopes
  • Immunologically active regions of immunogen that
    bind to antigen-specific antibodies or T-cell
    receptors

4
Antibodies (Abs)
  • Epitope binding proteins
  • Membrane bound on B cells OR
  • Secreted in blood
  • Humoral immunity
  • Share structural features, bind to antigen, and
    participate in number of effector functions
  • Known collectively as Immunoglobulins (Igs)
  • Abs dont kill anything, their job is to plant
    the kiss of death on an invader

5
T cell Receptor
  • T Cell Receptor
  • Expressed on surface of T cells
  • Recognize processed antigen complexed with MHC
    molecules

6
  • Immunogenicity
  • Ability to induce humoral and/or cell-mediated
    immune response
  • Immunogen is substance that induces response
  • Antigenicity
  • Ability to combine specifically with Abs or
    T-cell receptor/MHC
  • Not all antigens are immunogenic
  • Haptens

7
Haptens
  • Hapten too small, lack immunogenicity
  • If hapten is coupled to carrier protein, immune
    response can be induced
  • Hapten-carrier conjugate
  • Produces 3 types of antigenic determinants
  • Antibodies to hapten
  • Antibodies to carrier
  • Antibodies to hapten-carrier conjugate

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10
Properties of Immunogen contribute to
Immunogenicity
  • 4 Properties
  • Foreignness
  • Molecular size
  • Chemical composition and complexity
  • Ability to be processed and presented on MHC

11
  • Foreignness
  • Lymphocytes that do not bind to self antigens are
    allowed to further develop
  • Therefore they will later only recognized nonself
    antigens
  • For example
  • Bovine serum albumin (BSA) is not immunogenic
    when injected into cow but is when injected into
    chicken
  • Some macromolecules are highly conserved
    throughout evolution and display little
    immunogenicity
  • Cytochrome c, collagen

12
  • Molecular Size
  • Active (good) immunogens
  • gt 100,000 Daltons
  • Poor immunogens
  • lt 5,000-10,000 Daltons

13
  • Chemical Composition
  • Polymers composed of multiple copies of same
    amino acid or sugar tend to be poor immunogens
  • Lipids are haptens and need to be congugated with
    carrier to produce antibodies
  • Important for assays for detection of some
    steroids, vitamins

14
  • Susceptibility to antigen processing
  • Large, insoluble macromolecules are more likely
    to be phagocytized for processing

15
The biological system contributes to
immunogenicity
  • Host Genetic make-up
  • Manner in which material is presented
  • Use of agents (adjuvants) to enhance
    immunogenicity

16
  • Genotype of recipient animal
  • Genes of MHC
  • Genes in coding for specific antibodies

17
  • Material presentation immunogen dosage and
    route of administration
  • Too low or high of dosage can induce tolerance
  • Single dose is often not enough booster is
    needed
  • Route
  • Intravenous (iv)
  • Intradermal (id)
  • Subcutaneous (sc)
  • Intramuscular (im)
  • Intraperitoneal (ip)
  • Antigen administered iv would travel to spleen
    administered sc would travel to lymph nodes

18
  • Adjuvants
  • Enhance immunogenicity
  • Not exactly sure how they work but are recognized
    by Toll-like receptors
  • Water-in-oil adjuvants
  • Freunds incomplete adjuvant antigen in aqueous
    solution, mineral oil, and emulsifying agent
  • Antigen is then released very slowly from
    injection site
  • Based on Freunds complete adjuvant - also
    contained heat killed Mycobacteria

19
Epitopes
  • Antigenic determinants recognized by B cells and
    T cells
  • B cell epitopes tend to be on the outside of the
    antigen
  • For example, the hydrophilic amino acids on a
    proteins surface
  • T cell epitopes from proteins derived from
    enzymatic digestion of peptide and then
    association with MHC

20
Receptor-Ligand Interactions
  • Antigen receptors of the adaptive immune system
    are transmembrane proteins
  • B cells the B cell receptor
  • T cells the T cell receptor
  • Multiple noncovalent bonds
  • Hydrogen bonds
  • Ionic bonds
  • Van der Waals
  • Hydrophobic interactions

21
  • Receptor-ligand interactions induce signal
    transduction pathways
  • Translated to biochemical change within affected
    cell
  • Ligand binding can
  • Induce conformational changes in receptor
  • Alter receptor location within membrane
  • Phosphorylation is an early step in signaling
    pathways
  • Phosphorylation of certain amino acids on
    enzymes can activate or deactivate them
  • Phosphorylation of tyrosine on some molecules is
    seen early, serine and threonine later
  • PIP2 in cell membrane phosphorylated to PIP3,
    serves as binding site for other proteins in
    membrane
  • PIP2 also hydrolyzed by other enzyme to IP3 and
    DAG
  • IP3 interacts with endoplasmic reticulum
    vesicles, release of stored calcium, altering
    activity of other proteins
  • For example in lymphocytes, calcium ions bind
    calmodulin altering its conformation allowing
    dephosphorylation of NFAT (nuclear factor of
    Activated T cells)

22
Immunoglobulin Superfamily
  • All have similar structures
  • Examples
  • Antibodies
  • T-cell receptors
  • Class I and II MHC molecules
  • Part of B cell receptor
  • Most members of immunoglobulin superfamily cannot
    bind antigen

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25
Antibodies and B cell Receptor
  • B cell epitopes have characteristic properties
  • Located on surface of immunogen accessible to
    antibody
  • When talking about proteins, the epitopes can be
    sequential or nonsequential (referring to amino
    acid sequence) depending on protein folding

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Basic Structure of Antibodies
  • Known since late 19th century that antibodies are
    in serum
  • Serum is fluid phase that remains after plasma is
    allowed to clot
  • Antibodies are also found in other secretions

28
  • Antibodies are heterodimers
  • 2 light chains
  • 22, 000 daltons each
  • 2 heavy chains
  • 55,000 daltons each
  • First 110 aa of amino-terminal end of heavy and
    light chain vary depending on antibody specificity

29
  • Different digestion procedures reveal different
    fragments
  • F(ab)2 still shows antigen binding capability

30
Light Chains
  • When aa sequences of light chains from several
    individuals were sequenced, pattern emerged
  • Amino-terminal end (110 aa) varied
  • Other part remained constant
  • Were found to be either kappa (?) OR
  • Lambda (?)
  • In mice and humans, different lambda subtypes
    have been found

31
Heavy Chains
  • Amino-terminal end also shows variability
  • 5 different heavy chain constant regions
    (isotypes)
  • IgM µ
  • IgG ?
  • IgA a
  • IgD d
  • IgE e
  • Some subisotypes have been discovered in some
    species
  • Each antibody has 2 identical heavy chains, 2
    identical light chains

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Overall structure of immunoglobulin
  • Primary sequence of amino acids
  • Secondary folding into series of ß pleated
    sheets
  • Tertiary compact globular domains
  • Quarternary adjacent light and heavy chains
    interact

34
Secondary
35
Quartenary Structure
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38
  • Hypervariable regions complimentarity-determinin
    g regions (CDRs)
  • Complimentary to epitopes that they will bind

39
  • Ab-antigen interaction
  • Smaller antigens will fit in pockets in the
    variable regions of Abs
  • Larger antigens will interact with flatter
    regions of the variable region
  • 15-22 amino acid residues on antibody will
    interact with residues on antigen

40
  • Hinge Region
  • ? (gamma), d (delta), and a (alpha) heavy chains
    have extended peptide sequence
  • Rich in proline and cysteine
  • Gives flexibility
  • Immunoglobulins can be secreted or membrane-bound
  • Membrane-bound differ in the carboxyl-terminal
    end
  • Extracellular spacer of 26 aa
  • Hydrophobic transmembrane sequence
  • Cytoplasmic tail

41
B Cell Receptor (BCR)
  • Heavy chain portion of membrane-bound antibody
    does not extend far enough through the cell
    membrane for signaling
  • Membrane bound antibody is accompanied by Iga and
    Igß

42
Antibody-mediated Effector Functions
  • Remember, they plant kiss of death on an
    invader
  • In addition to binding antigen, Abs can
  • Promote phagocytosis (opsonization)
  • Activate complement
  • Antibody dependent cell mediated cytotoxicity
    (ADCC)
  • Natural killer cells have receptor for Fc portion
    of antibody
  • Some can cross epithelial layers to be excreted
    through mucous or across placenta

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  • Monomeric IgM expressed on B cells
  • Secreted is pentameric
  • 1st class produced in primary response
  • Activates complement
  • Very good at agglutination
  • 5 times the binding sites

46
  • Membrane bound on B cells

47
  • Most abundant
  • 4 human subclasses
  • Crosses placenta
  • Involved in complement
  • Gives us our immunity

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  • Involved in allergic reactions
  • Involvement in parasitic infections

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51
  • Predominant class in secretions
  • J chain and secretory component helps with
    transport across intestinal wall
  • J chain makes IgA more resistant to acids and
    enzymes found in digestive tract
  • IgA and macrophages restrict commensal bacteria
    that occasionally enter the tissues from the
    intestines
  • Better for IgA to interact than IgG this is
    because the Fc portion of IgG has high affinity
    for receptors of immune cells and would
    constantly trigger inflammatory responses
  • Can cross-link large antigens
  • Exists as dimer

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  • Immunoglobulins when injected into another
    species can be immunogenic
  • Isotypic differences in constant region from
    one species to another
  • Allotypic differences (alleles) that occur in
    some individuals
  • Idiotypic differences in variable regions will
    differ even on Abs of same isotype

54
Monoclonal Antibodies
  • Most antigens offer multiple epitopes
  • However, a single B cell will only produce
    antibody specific to single epitope
  • Antibodies found in serum are from many different
    B cells
  • Polyclonal antibodies
  • However, for diagnostic uses, monoclonal
    antibodies are needed

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T cell receptor vs B cell receptor
  • T cell receptor is only membrane bound
  • Doesnt appear in soluble form like antibodies so
    more difficult to assess its structure
  • Antigen binding of T cell receptor is weaker than
    that of antibodies
  • Antigen recognized by T cells is not antigen
    alone but antigen associated with MHC molecules

57
  • (a) T cell receptor (TCR) is specific for peptide
    A
  • (b) Right MHC haplotype but wrong antigen
    (peptide B)
  • (c) Right antigen (peptide A) but wrong haplotype

58
  • T cell receptor (TCRs)
  • TCR heterodimers are similar to immunoglobulins
  • Therefore they are classified in immunoglobulin
    superfamily
  • Resembles Fab fragment

59
  • TCRs
  • Associate with MHC aß TCR
  • Do not associate with MHC ?d TCR
  • Much remains to be learned of function of ?d TCR

60
TCR-CD3 Complex
  • Accessory molecules help in signal transduction
    after interaction of T cell with antigen
  • 2 Zeta ?? chains
  • Heterodimer of delta epsilon ?e chains
  • Heterodimer of delta epsilon ed chains

61
T cell accessory molecules
  • T cells can be divided into 2 populations
  • CD4
  • Recognize antigen associated with Class II MHC OR
  • CD8
  • Recognize antigen associated with Class I MHC
  • CD4 and CD8 function as coreceptors and assist
    with signal transduction

62
  • Affinity of TCR for peptide-MHC complexes is
    enhanced by coreceptors

63
  • Allogenic genetically different individuals of
    same species
  • Alloreactivity of T cells is puzzling
  • Evidence supports that T cells can only respond
    to antigenMHC
  • However, T cells can recognize a foreign MHC
    molecule alone
  • As with transplants
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