Immunoglobulin:Ab-Ag Interactions

Lecture 10 20/10/2015

Antibody structure

Human Ig classes

Types of Ig variation

• Isotype• Allotype• Idiotype

Isotypic variation ( Iso, the same). The genes for isotypic variation

are present in all members of a given species. Examples include

alternative or alternative forms of light chain and the different

heavy chains 1 2 3 4 1 2 and .

Isotypic variation

IgM (kappa)

IgG1 (kappa)

Allotypic Variation (allos, other) This refers to variation in a

population present in some individuals but not others. This variation

is encoded in alternative alleles of a given gene . Most Ig allotypes

are due to single amino acid differences particularly in the heavy

chain gene sequences.

Allotypic variation

IgG1 (kappa)

Person 2

IgG1 (kappa)

Person 1

Idiotypic variation (idios, own or personal). Differences in structure of

the antibody variable region or Ag binding site give rise to idiotypic

variation . Hypervariable regions contribute strongly . Abs of the same

class but different specificity will have different idiotypes.

Idiotypic variation

IgG1 (kappa)

Person 1

anti-B

IgG1 (kappa)

Person 1

anti-A

Summary

Nature of Ag/Ab Reactions

• Lock and Key Concept

• Non-covalent Bonds

1. Hydrogen bonds2. Electrostatic bonds3. Van der Waal forces4. Hydrophobic bonds

• Reversible

• Multiple Bonds

http://www.med.sc.edu:85/chime2/lyso-abfr.htm

Ab/Ag interactions

Binding of an antibody to a specific antigen is one key

characteristic of the antibody molecule. How does

this occur?

In vivo Most antibody responses are polyclonal.

That is we respond by producing a range of

antibodies each with different antigen binding sites.

These antibodies can bind to different antigenic

determinants on or areas of a protein or to several

of the proteins in an complex antigen.

Polyclonal antibody

Clonal selection hypothesis

+

Stem cell (bone marrow)

Large pool of B cells each with Ab of one specificity IgM or D

Ag Antigen selection

Proliferation and differentiation

Antigen specific B cells Specific antibody

ANTIGENS, ANTIGENIC DETERMINANTS AND EPITOPES

Antigens

1. Foreign molecules which generate antibodies are called antigens >10kd

2. Pathogens, micro-organisms, proteins

3. Surfaces are antigenic determinants, epitopes, or molecular shapes. Few ~5 Amino Acids in size, or CHO molecule .

Repeated antigenic structure eg bacterial cell wall

Differing antigenic structure eg eukaryotic proteins eg albumin

Types of Epitopes

Forces in Ab/Ag Interactions

1. All non-convalent

i) Hydrogen bonding

ii) Electrostatic forces

iii) Van der Waals forces

iv) Hydrophobic bonds

Forces (continued)2. Complementary Shape and Position.

Critically dependant on distance.

Close and multiple in number.

Requirement for complementary structure:

a) suitable atomic groups must be together

b) shape of site must fit the determinant

Factors Affecting Measurement of Ag/Ab Reactions

1. Affinity

2. Avidity

3. Ag:Ab ratio

4. Physical form of Ag

Ab excess Ag excess

Equivalence – Lattice formation

Refers to the interaction of:

1. Monovalent Ag or single determinant.

2. Single Ab combining site

3. Product of the summation of forces.

4. Ab + Ag AbAg

A measure of the binding strength between a single

antigenic determinant and a single Ab combining site

Antibody Affinity

1. Ab are multivalent (IgM)

2. Multivalent Ag

3. Functional affinity

4. Bonus effect of multivalency

Avidity is not however the sum of the individual binding site affinities.

In practice the divalency of IgG increase its binding to a multimeric Ag by around 104 times.

The pentameric structure of IgM to around 107 times.

Antibody Avidity

In practice the divalency of IgG increases its binding to a multimeric Ag by around 104 times.

The pentameric structure of IgM to around 107 times.

Affinity Avidity IgGAvidity IgM

104 x greater107 x greater

Cross Reactivity• The ability of an individual Ab combining site to

react with more than one antigenic determinant.• The ability of a population of Ab molecules to

react with more than one Ag

Anti-A Ab

Ag A

Anti-A Ab

Ag B

Shared epitope

Anti-A Ab

Ag C

Similar epitope

Cross reactions

The End