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Lecture 26

MIMM 214 Lecture 26: Adaptive Immunity IX: Antibodies

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McGill University
Microbiology and Immun (Sci)
MIMM 214
Claire Trottier

Lecture 26 Friday, March 10, 2017 Antibodies Structure of an Immunoglobulin (antibody) • Y shaped glycoprotein • Made up of 2 identical disulfide-linked heavy (H) chains and 2 identical light (L) chains o Names dependent on the relative sizes of the chains • Each chain uses Ig-like domains • 4 chains in total • Variable regions - both the heavy and light chain have a variable region • Constant regions - both the heavy and light chain have a constant region • Variable regions function: Ag binding o V Lnd V Horms antigen-binding site (2 identical sites per antibody) o Binding can result in neutralization (not an absolute) • Constant regions function - effector functions: o Complement activation o Constant region (Fc) can bind to Fc receptors on phagocytes and other cell types • E.g. mast cells, eosinophils • Papain o Cleaves antibody near the disulphide bonds that connect the two heavy chains o Cleaves Ab into 3 molecules • Fab fragments o Two Fab fragments per antibody o Each has antigen-binding binding domain AND part of the constant H and L chains o Fab = fragment antigen binding • Fc fragment o One Fc fragment o Fc = fragment crystallisable o Receptors that bind antibodies recognize the Fc region - called Fc receptors • Immunoglobulin = antibody o Two heavy chains (variable and constant regions) o Two light chains (variable and constant regions) o Held together by intra-/interchain disulfide covalent bonds • Three dimensional structure o Both the constant and variable regions are folded in complex 3D structures, including β strands o Disulphide bonds help hold these structures in place Variable Region • Complementarity determining region (CDR) o Antigen-binding site o Hypervariable loops • Three loops per variable domain o Not part of the β strands • CDRs are highly variable • Compare the location of the hypervariab
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