Humoral immunity and antibodies

Complementarity determining region (CDR)

The part of the antibody molecule providing the complementary mirror image to an epitope. Formed from the 6 hypervariable regions in VhVL

Hapten

Small organic molecules which is not immunogenic but contribute to binding. Immunisation with hapten + carrier creates new epitopes that stimulates hapten specific Ab

examples

nickel, penicillin, dinitrophenol

Epitope

• are defined by antibody, there is no epitope with without antibody

• The complete region on an antigen encompassed by the Ab binding site

• Antigens can have multiple epitopes, the more complex the antigen, the more epitopes it have and the more immunogenic it is. Size does matter.

Ig amino acid sequences have conserved and …

Where are the conserved cysteine residue on the framework regions?

highly variable regions: encoded by the V(D)J gene segments in the heavy chain and VJ segment

always cysteine at 24 and 89 position

25 kDa immunoglobulin light chain

one of the two types of polypeptide chains in an antibody

Composed of one variable domain (VL) and one constant domain (CL)

The three hypervariable loops CDR are on the variable domain (VL)

The variable (V) domain is composed of 9 b-strands which provides a stable scaffold for the CDR loop

The constant (C) domain is composed of 7 b-strands , providing structural support

Pairs with a heavy chain to form one half of the antigen-binding site

What is the molecular weight of IgG?

150kDa

What are the 6 important antibody functions?

1. Opsonisation

2. Block adherence: stop bacteria/ viruses adhering to mucosal cells for entry. IgA is the best for this because it is secreted for mucosal immunity

3. Neutralise: direct binding to toxins to block receptor binding

4. Agglutination: clumping cells together as a result of multivalent antibdy

5. Immobilisation: eg, binding to bacterial flagellum

6. ADCC- antibody dependent cell cytotoxicity: antibodies binds target on cells and trggers NK clls via Fc receptor binding

IgM molecular weight and functions

900 kD

default isotypes made by all B cells

5 chains joined by J cahn

up to 10 binding sites for antigens

low affinity but very high avidity

reacts best to surfaces (ie, microbes)

powerful opsonin

fixes complement

IgG molecular weight and function

most abundant n serum

crosses placental for neonatal immunity

anti-toxin (soluble) as well as anti-bacterial (surface)

opsonises

blocks receptor binding

high affinity and high specificity

product of affinity maturation

IgA

160-300 kD

first line of defense at mucosa

blocks pathogen adhesion

secreted form linkd by J chain and secretory components (protects and transports across the epithelial barrier

IgE -200 kD

Least abundant in blood

produced by plasma B cells in mucosa (lungs, guts)

Defense against large organism (parasites) and complex antigens (eg, pollen)

Potent activator of mast clls

High affinity for FceR receptor on mast cells

• Type 1 hypersensitivity

• Atopic allergy

• Allergy and anaphylaxis

What are the molecular forces that govern AbAg binding?

Hydrogen bonding

Electrostatic

Van der Waals

Hydrophobic

All reversible and non-covalent

First order kinetics

Affinity vs avidity

affinity: the strength of a single binding interaction between the antibody’s paratope and an antigen’s epitope

avidity: the overall binding strength of a multivalent antibody to a multivalent antigen

eg, even only with IgG, IgG bound to unfixed antibody would have an affinity, whereas binding to fixed antigen that has 2 valance would be avidity, which increase the cummulative strength