Immunology 7 - Immunology 7: Immunoglobulins...

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Immunology 7: Immunoglobulins Antibodies=secreted form of the B-cell receptor; soluble and secreted into the blood Y-shaped; 3 equally sized portions connected by a flexible tether; variable region=arms (involved in antigen binding), constant region=stem (interacts with effector cells and molecules 5 classes of immunoglobulins: IgM, IgD, IgG, IgA and IgE—distinguished by their constand region IgG antibodies and B cell receptors consist of four polypeptide chains linked by disulphide bonds Composed of 2 different kinds of polypeptide chains: heavy chain (50kDa) and light chain (25kDa) ; each IgG molecule consists of 2 heavy and 2 light chains (total 150kDa) 2 heavy chains linked by disfulide bonds; each heavy chain linked to light chain by disulfide bond The 2 heavy chains and 2 light chains are identical in a given immunoglobulin molecule→gives antibody 2 identical antigen binding sites (bind simultaneously 2 identical antigens on the surface, therefore increase the total strength of the interaction= avidity ) Affinity= strength of the interaction between a single antigen-binding site and its antigen 2 types of light chains: lambda and kappa found in antibodies; given immunoglobulin either has lambda or kappa chains, NEVER ONE OF EACH; ratio varies from species to species; distortions used to detect abnormal proliferation of a clone of B cells; (1:2 in humans—kappa is more frequent) The class, and thus the effector function, of an antibody is defined by the structure of its heavy chain 5 main heavy chain classes (isotypes): IgM (µ), IgD (δ), IgG (γ), IgA (α), IgE (ε); heavy chains denoted by greek letter IgG=most abundant and has several subclasses (IgG1, 2, 3, 4) Distinctive functional properties of the different classes and subclasses of antibodies are conferred by the carboxy terminal part of the heavy chain (where it isn’t associated with the light chain) General structural features of all isotypes are similar Structure of B cell receptor is identical to that of its corresponding antibody except for a small portion of the carboxy terminus of the heavy-chain C region; B-cell receptor: carboxy terminus is a hydrophobic amino acid sequence to anchor it to the membrane; antibody: its a hydrophilic sequence that allows its secretion Immunoglobulin heavy and light chains are composed of constant and variable regions Each chain consists of a series of similar, although NOT identical, sequences (110 aa long); each of these repeats corresponds to a discrete, compactly folded region protein structure known as a protein domain Light chain: made up of 2 such immunoglobulin domains Heavy chain: made up of 4 immunoglobulin domains *immunoglobulin chains have evolved by repeated duplication of an ancestral gene corresponding to a single domain Amino-terminal amino acid sequences of the heavy and light chains vary greatly between antibodies; variability limited to approximately the first 110 aa (corresponding to 1
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Immunology 7 - Immunology 7: Immunoglobulins...

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