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- 1The immunoglobulin Domain- functional unit associated with recognition and includes: an internal disulphide bond. Heavy chains of antibodies consist of 4x (Ig) domains whereas antibody light chains have 2x Ig domains arranged into antiparallel β-pleated sheets
- Each Ig domain is encoded by a exon - 2Specific immune recognition - Antibody structureAn antibody or immunoglobulin (Ig) is a Y-shaped molecule and consists of dimer of dimers
- 3Antibody structureTwo L chain gene classes exist termed kappa ( ) lambda (λ ); in a single antibody molecule either or λ chains are used, both are NOT found in same antibody.
H chains determine the class/subclass of the antibody made. - 4Antibody structureThe N-terminal Ig domains of H and L chains termed VH & VL variable – these differ between individual Ig molecules
The other domains are called constant region domains (C) they do not differ between antibodies within a species. - 5Cleavage of Ig by papainLimited treatment of Ig with papain, a cysteine protease, cleaves a single Ig molecule into three fragments adjacent to the hinge and creates two Fab (fragment antigen binding) and 1x Fc (fragment crystallisable).
- 6Antigen-binding by the antibodyAntibodies bind to molecules that are foreign – molecules may be free in solution or attached to (intrinsic to) a pathogen. The molecules that antibodies bind are termed antigens – nearly always macromolecules
- 7Epitopes are antibody binding sitesAn antibody does not bind to the entire surface of a macromolecule (antigen), rather it will target a particular chemical structure termed an epitope that is likely to range from 8 to 22 amino acids.
Epitopes can either be linear or be derived from a particular folding pattern - 8The antigen-binding site of antibodiesThese are located on the variable domains of the H and L chains. The antigen binding sites are formed from the quaternary structure of the L and H chains. Antibodies that recognise different antigens have a different sequence in the VH and VL domains.
- 9Hypervariable regions of IgRegions binding epitopes are confined to 3 segments each of 5-15 amino acids within each heavy and light chain these are termed the hyper-variable regions of the antibody. They contribute to the overall 3-D structure
- 10Chemical bonds between epitope and IgThese are non-covalent and involve weak interactions-H-bonds, electrostatic interaction, van der Waals, and hydrophobic forces.
The strength of the interaction is dependent upon a “good fit” and the strength of the interaction is termed affinity. - 11Classes of antibodyIn human there are 5 classes IgM, IgG, IgA, IgE, IgD, determined by the structure of the Fc domain.
Different classes of antibody have different functions
The specificity of an antibody for a target antigen is not related to its class. - 12—IgM – Pentamer, 4 constant domains for H-chains and J-chain for polymerisation
IgA – Secretory piece facilitates antibody secretion from epithelial cells.
IgE – 4 H-chain constant domains, involved in asthma/allergy.
IgD – function not clear
IgG – most prevalent Ig class
- 13Generating antibody diversityDifferent sets of antibodies recognise different antigens and these are designed and constructed for each antigen rather than taken “off-the-shelf”.
- 14Requirements for antibody production:Only make antibodies specific for the antigens exposed to
Make different classes of antibody with different functions
Make antibodies with higher affinity – these can be at a lower concentration - 15Clonal expansion/clonal selectionAntigen binding to a particular B cell bound Ig molecule will initiate clonal expansion of that cell – also termed clonal selection.
Binding alone is not sufficient, but is necessary for clonal expansion. - 16Antibody class switchInvolves switching from IgM and IgD to IgG, IgA and IgE, using a mechanism that changes the heavy chain constant region whilst keeping the same variable domains.
- 17Molecular basis of Class switchingDNA of unswitched B-cell
IgM+D are made using Cμ+C
DNA of switched IgG1 B-cell, V region is upstream of Cγ1 – IgG1 is made - 18Cytokines control class switchingIgA dependent on Il-5/TGFbeta
IgE requires Il-4/IL-13
IgG (subtypes) Il-4/IL-13 (1), IFNgamma/TGFbeta (2), TGFbeta (3) - 19Final stage of B cell differentiationB cells differentiate into (1) plasma cells that secrete large amounts of antibody
B cells differentiate into (2) memory B cells that retain surface bound immunoglobulin with the same specificity of the original cell to provide long term protection – memory response on subsequent exposure to pathogen carrying that antigen – desired response of vaccination - 20Antibody responseDisplays 4 characteristic attributes:
Antigenic specificity
Diversity
Immunological memory
Self/non-self recognition
Antigenic specificity- to discriminate a single amino acid difference between two protein molecules. - 21Antibody-mediated responses-humoral immunityAntibody can protect against infectious organisms in a number of ways
Neutralization
Agglutinization
Opsonisation
Activation of complement
Antibody-dependent cell-mediated cytotoxicity - 22Neutralization- Viruses, bacteria (or toxins) need to interact with cells (receptors) for internalization
- Antibody binding to foreign organism prevents receptor binding
- Antibody binding to toxin inhibits its activity - 23Agglutination – limits bacterial spreadingAntibodies are bivalent – 2 binding sites except IgM/A
Clumps of microbes more likely removed by phagocytosis
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- Highschool Biology Karteikarten am IMP3, erstellt von jade Belboda am 02/06/2015.
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