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Antibodies and antigens
Antibodies = immunoglobulins
Antibodies bind antigens.
3 different kinds of antigen-binding
molecules; Ig, TCR and MHC.
Antibodies
•On the surface of B cells: Antigen receptors for activation of B cells.
•Secreted: Effector phase of humoral immunity. Cooperation with complement, phagocytes, mast cells and eosinophils.
Distribution of secreted Ab
•Plasma
•Mucosal secretions
•Interstitial fluid
•Surface of immune effector cells
Antiserum: Serum with specific ab.
Titer: Reciproke of end point dilution.
End point dilution: Highest dilution of serum that gives a positive reaction.
Dilution: 1/100 1/200 1/400 1/800
Reaction: + + + −
End point dilution: 1/400
Titer: 400
Effects of antibodies:
•Neutralization of microbes or toxins
•Complement activation
•Opsonization
•Antibody dependent cytotoxicity
•Immediate hypersensitivity
Antibodies are clonally distributed:
Clone: Population of identical cells, derived from a single cell by cell division.
Clonal distribution of abs:
One clone makes only one ab, other clones make different abs.
Allelic exclusion.
Serum: Polyclonal Ab
Myeloma tumors: Monoclonal Ab
Hybridomas
Detection of monoclonal bands by serum electrophoresis in agarose gel.
+
Application -
Albumin
γ globulin
Hybridoma monoclonal antibodies
Some advantages of monoclonal abs:
•Increased specificity
•Unlimited supply
•Standardized reagents; can compare results from different labs.
•Possible to immunize with impure antigens, e.g. whole cells.
Structure of antibody molecules:
H2L2 (1, 2 or 5; monomer, dimer or pentamer)
(H: Heavy chain)
(L: Light chain)
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Structure of an Ab light chain
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The immunoglobulin superfamily
Proteolytic fragments of an IgG molecule
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(Rabbit)
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Hypervariable regions in Ab molecules
Variability: No. different aa/Frequency of most common aa.
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Binding of antigen by an antibody’s hypervariable regions of VH and VL.
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Flexibility of antibody molecules
Antibodies are divided in classes (isotypes) based on differences in heavy chains.
Human Ig
classes
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Membrane and secreted forms of Ig heavy chains.
Two types of light chains:
κ (kappa)
λ (lambda)
Synthesis of Ig:
Rough endoplasmic reticulum
Chaperones (calnexin, BiP) ensure proper folding
Golgi complex
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Ig expression during B lymphocyte maturation.
Antigens bound by Abs:
Proteins
Carbohydrates
Nucleic acids
Phospholipids
Small chemicals
Haptens: Small molecules that must be conjugated to carrier molecules (proteins) to initiate an immune response. They are antigenic (bind Ab) but not immunogenic (cannot by themselves induce an Ab response).
Immunogens are macromolecules, and (of course) antigenic.
Haptens and carriers
(Bogen and Munthe)
(Carrier molecule)
(Hapten-carrier complex)
(not)
(to)
Determinants (epitopes)
Poly (multi) valent antigens
Polyvalent antigens can be immunogenic for B cells without the help of T cells
Nonoverlapping determinantsOverlapping determinantsAllosteric effectsNeoantigenic determinants (result of postsynthetic modifications, such as proteolysis or phosphorylation)
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The nature of antigenic determinants
Antibody-antigen binding: Noncovalent forces
•Electrostatic forces
•Hydrogen bonds
•van der Waals forces
•Hydrophobic interactions
Avidity and valency of antibody-antigen interactions
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Bogen and Munthe
Avidity
Affinity
Equilibrium dialysis
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Antigen-antibody complexes
Three important characteristics of humoral immunity:
Specificity
Diversity
Affinity maturation
•Specificity (distinction of similar antigens)
Karl Landsteiner
Meta-azoben-zene sulfonate
Cross-reactions
•Diversity (specific binding of a large number of antigens).
•Affinity maturation (late abs and abs in secondary reactions bind better).
Changes in antibody structure during humoral immune responses
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Isotypes
Allotypes
Idiotypes
Binding of two or more adjacent Fc portions is needed to trigger effector functions such as complement activation and phagocytosis.
Effector functions often mediated by Fc portion.
Effector functions are initiated by ab that has bound ag.
Isotypes influence how microbes are attacked.
Isotypes determine tissue distribution.