Antibodies and antigens Antibodies = immunoglobulins Antibodies bind antigens

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.