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 bonds 2. Electrostatic bonds 3. Van der Waal forces 4. Hydrophobic bonds Reversible Multiple Bonds

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 cellsSpecific 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 10 4 times. The pentameric structure of IgM to around 10 7 times. Antibody Avidity

In practice the divalency of IgG increases its binding to a multimeric Ag by around 10 4 times. The pentameric structure of IgM to around 10 7 times. Affinity Avidity IgGAvidity IgM 10 4 x greater 10 7 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