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MICR 304 Immunology &
Serology
MICR 304 Immunology &
Serology
Lecture 7B Antibodies Part IChapter 3.1 – 3.9
Lecture 7B Antibodies Part IChapter 3.1 – 3.9
Overview of Today’s Lecture
• Basic structure of antibodies • Generation of antibodies• Structural variations in the
constant regions
Key Players in Immunology
Innate Adaptive
Cells PhagocytesEpithelial Cells
NK Cells
Lymphocytes(B-Ly, T-Ly)
Effector Molecules
ComplementAntimicrobial (Poly)PeptidesAntimicrobial
lipids?
Antibodies
What is an Antibody?
• Glycoprotein• Binds to antigen• Consists of 2 heavy chains
() and 2 light chains () connected by disulfide bonds
• Variable domains (antigen binding)
• Constant regions (effector function)
Antibody Classes
• Determined by the type of constant chain: IgG: IgM: IgA: IgE: IgD
Used to explain principle make up of an ab
Basic Structure of an Antibody Molecule
• 2 light and 2 heavy chains
• Disulfide bonds• Hinge region• N-terminus: variable,
antigen binding• C-terminus: constant
region, effector function
Proteolytic Cleavage of the Antibody Molecule
•Variable regions intact•Antigen binding without effector function
•Constant regions intact•Crystallizable
•Variable regions intact and additional amino acids•Antigen binding without effector function
Single site
Multiple sites
pepsin
Antibody Molecules are Flexible
SpikesDisappear
after treatment with pepsin
Antibody arms are joined by a flexible hinge.
The Structure of Immunoglobulin Constant and Variable Domains
Immunoglobulin superfamily domain(4 strand + 3 strand) of (4 strand + 5 strand)
Found in many other proteins
Anti-parallel strands forming 2 sheets
that assume a barrel structure
V domain is larger!
Localized Regions of Hypervariable Amino Acid Sequences Form the Antigen Binding
Site
• Hypervariable regions (HV1-3) of light chain and heavy chain create the antigen binding site– Surface complementary to
the antigen – Complementarity-
determining regions (CDRs)
– Combinatorial diversity• Framework regions (FR1-
4) provide structural frame work
Variability Plot
Compares aa sequences of many different V regions
Hypervariable Regions Lie in the Discrete Loops
• Different aa in different CDRs form different surfaces and bind different antigens.
Antigen Binding Sites Assume Varying Shapes
Pocket Groove Extended Surface
Pro
tru
din
g S
urf
ace
Hapten
• Antigen that is too small to elicit antibody response by itself
• When coupled to carrier protein antibodies can be generated
• Once antibodies are generated they can recognize and bind to uncoupled hapten
Epitop• Domain on antigen which
actually binds to antibody binding site
• Antigenic determinant• Two types of epitops
– Conformational• discontinuous aa sequence
– Linear• continuous aa sequence Y Y
Y
Y
Non-Covalent Forces inAntigen-Antibody
Interactions
(Salt bridges)
(Electric dipoles)
Short distance
Overall interaction
Antibody-Antigen Interactions
• Never covalent!• Reversible• Depends on the actual antigen
and antibody• Single amino acid changes can
cause loss of recognition
Lysozyme
Heavy Chain
LightChain
Glutamine residue (Ly)making hydrogen bonds
Active Learning Exercise
• How can the interaction between antibody and antigen be disrupted and antigen released from the antibody?
Today’s Take Home Message• The IgG antibody molecule consists of 2 identical
light chains and 2 identical heavy chains connected by disulfide bridges.
• Each chain contains at the N-terminus a variable region for antigen binding and at the C-terminus a constant region for effector function.
• The variable regions of the light and heavy chain form antigen binding sites.
• Each antibody molecule has two identical antigen binding sites allowing for cross linking of antigen.
• Hypervariable loops comprise the complementarity determining regions that form the surface interacting with the antigen