MICB 212 Chapter 6 Notes

The Structure and Function of Antibodies

-antibody is secreted form of B cell receptor

-recognition of antigen mediated by 2 antigen binding sites of variable region

-constant region of heavy chain determines antibody’s biological role in immune response

Antibody Structure

-all immunoglobulin molecules consist of 4 polypeptide chains

  • 2 identical H chains

  • 2 identical L chains

-some antibodies exist as dimers or pentamers of immunoglobulin molecules

L Chain Types

-kappa (κ) chain

-lambda (λ) chain

H Chain Types

-gamma (γ) chain

-mu (μ) chain

-delta (δ) chain

-epsilon (ε) chain

-alpha (α) chain

-type of H chain defines immunoglobulin class or isotype

  • immunoglobulin may have κ chain or λ chain L chain, but never both

Variable Regions

-N-terminal portions of both H and L chains are different for different antibodies → variable (V) regions

Hypervariable Regions

-within variable regions of immunoglobulins → 3 short stretches of amino acids that vary most from one antibody to another

  • hypervariable (HV) regions

-H, L chain V regions (HVand HVL) contribute significantly to antigen-binding site are part of antibody that contacts the antigen

Constant Regions

-rest of H and L chains are constant (C) region

-each H chain (μ, δ, γ ε, α) has a different amino acid sequence in its C region

  • determines function of antibody

-different isotypes of antibodies have different roles in immune system

-C regions of L chains have different amino acid sequences

  • but doesn’t influence function of antibodies

-difference in amino acid sequence of C regions of heavy chains. between species is significant

  • can cause a clinical setting when human patient needs to be transfused with antibody from non-human species

Antigen-Binding Site

-single antibody molecule has

  • 2 identical H chains

  • 2 identical L chains

→ antibodies are bivalent

  • single antibody molecule has 2 antigen-binding sites

-antigen-binding site is a 3D pocket

  • side chains of amino acids make up H & L chain hypervariable regions determine shape of antigen-binding site

  • shape of antigen-binding site determines specificity of antibody

-single antibody molecule binds to only small portion of antigen molecule (epitope/antigen determinant)

  • epitope has 3D structure

  • antigen-binding site of antibody has shape that is complementary to epitope

  • epitope on antigen fits in antibody’s antigen-binding site like key and lock

-different antibodies have antigen-binding sites with different shapes

  • bind epitopes with different 3D structures

-single antigen can have many epitopes

  • there can be many copies of same epitope, different epitopes or both

-epitopes on surface of antigen are accessible to antibody

  • many antibodies with different specificities can bind to their particular epitope on one antigen particle

  • possible for 2 different antigens to have an epitope in common

-antibody binds to epitopes with a complementary shape

  • close contact between surface of epitope and surface of antigen-binding site is required for strong binding

-poor fit suggests there is weak binding or no binding

-close contact allows formation of H bonds, ionic and hydrophobic interactions between epitope and antigen-binding site

-single antibody will bind only one or a few similarly shaped epitopes

Secreted Antibodies

-IgM, IgA, IgG are main types of secreted antibodies

-made by activated B cells

-IgD not secreted

  • only found on cell surface of naive B cells

-antibodies of different isotypes can recognize same antigen because they have same V region

-secreted IgM is a pentameter consisting of 5 IgM monomers that are joined to each other by disulfide bonds

  • 2 IgM monomers are connected by a polypeptide (J/Joining chain)

Secreted IgM

-low binding affinity

-pentameric IgM has high avidity (total binding strength)

  • 10 binding sites instead of 2

-more binding sites = greater binding strength

-amino acid sequence in μ chain constant region binds efficiently to complement proteins

  • bacteria coated with IgM killed when complement is activated

-able to neutralize pathogen

Secreted IgG

-found in blood

-incapacitate pathogens by neutralizing them

-kills bacteria in 2 ways

  1. complement proteins kill bacteria coated with IgA

    • amino acid sequence in γ chain constant region binds effectively to complement proteins

  2. phagocytes ingest bacteria coated with IgG antibodies more readily than bacteria without IgG

    • amino acid sequence in γ chain constant region facilities binding of IgG to specific receptors on phagocytic cells

-IgG antibodies from mother can cross placenta to protect fetus

Secreted IgA

-in serum — monomer

-in bodily secretions — dimer

-J chain joins 2 monomer IgA units

-dimeric IgA in bodily secretions associated with another polypeptide (secretory chain)

  • secretory chain help IgA cross layers of epithelial cells and get to appropriate body sites

-IgA is main immunoglobulin class in bodily secretions

  • bind to and neutralize pathogens

  • prevents attachment of pathogens to host surfaces

Secreted IgE

-secreted into serum

  • almost all binds immediately to mast cells and basophils

-not available in serum to neutralize pathogens

-amino acid sequence in ε chain constant region binds to receptor on these cells

  • IgE is the only antibody class that can bind to this receptor (FCεRI)

-responsible for triggering allergic reactions

-when IgE on mast cell or basophil binds an antigen

  • cell degranulates and releases large amounts of histamine

-main class of immunoglobulin induced in response to infection by parasites such as intestinal worms

  • worms are too big to be killed via phagocytosis

  • mast cell degranulation results in diarrhea and vomiting to expel the worms

Immunoglobulin Class Switching

-B cell uses a different CH region

  • class and function of Ig is different

  • antigen specificity remains the same

-first antibodies produced in immune response always IgM

  • activated T helper cells may induce some B cells to switch to a different class (isotype) of Ig by providing specific cytokines

-class switching is an important method of tailoring adaptive response to best match antibody to pathogen

-immunoglobulin class switching involves cutting out DNA

  • B cells may undergo class switching on more than one occasion

  • mechanism of switching is uni-directional

    • B cell can’t be switched back to original Ig type after splicing because DNA is lost

-some B cells that undergo immunoglobulin class switching may differentiate into long plasma cells and secrete the new class of Ig

  • other cells may become long-lived memory B cells

Functions of Secreted Antibodies

-antibodies protect against pathogens

  1. prevent viruses and toxins from binding to cell surfaces and entering cells (neutralization)

  2. coat virus and bacteria, improve efficiency of phagocytosis by neutrophils & macrophages (opsonization)

  3. allow more types of bacteria to be killed by complement proteins (complement activation)

Complement Activation by Antibodies

-alternate pathway can activate complement after complement C3b binds to surface of pathogen

  • complement proteins are regarded as part of innate response → they still work with IgM and IgG antibodies generated during adaptive immune response

-after IgM and IgG has bound to surface of pathogen

  • early complement proteins are recruited

    • formation of C3 convertase

  • C3c cleaves C3 to C3a and C3b

    • binding of C3b to pathogen surface recruits additional proteins to form C5 convertase which cleaves to C5a and C5b

    • C5 is also deposited on pathogen surface and initiates assembly of late proteins to form membrane attack complex (MAC)

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