Immunology 4: Antibodies

Question 1

What produces antibodies

Answer 1

b lymphocytes

Question 2

after binding antigen, antibodies initiate what three secondary effector functions

Answer 2

complement activation

opsonisation (promotion of phagocytosis)

cell activation via specific antibody-binding receptors (Fc receptors)

Question 3

What immunoglobulin catergory are antibodies in

Answer 3

Antibodies are in the GAMMA IMMUNOGLOBULIN category

Question 4

What is the shape of an immunoglobulin

Answer 4

N, C = polypeptide terminals

4 polypeptide chains held together by disulphide bonds

The two heavy chains are identical and the two light chains are identical

SYMMETRICAL

Question 5

What is the class of protiens that antibodies fall under

Answer 5

immunoglobulins - a large family of soluble glycoproteins

Question 6

Are all antibodies completely different?

Answer 6

Both light chains and heavy chains can be divided into Variable and Constant regions.

The variable part is where the antigen binding site is so the amino acid sequence must vary to give the antibody different specificities.

The constant part is the Fc part which doesn’t have to be variable because it does not bind to antigens.

Question 7

Where are disulphide bonds found in the antibodies

Answer 7

Between chains and interchains

The interchains hold together immunoglobulin domains

Question 8

What are the hypervariable regions

Answer 8

located at loops which are at the end of the protein and interact with antigen.

THREE REGIONS which are HYPERVARIABLE and they are called the Complementarity Determining Regions

Question 9

Forces involved in antibody-antigen binding

Answer 9

Hydrogen Bonds

Ionic Bonds

Hydrophobic Interactions

van der Waals interactions

Question 10

What is affinity

Answer 10

The strength of the total non-covalent interactions between a single antigen-binding site and a single epitope on an antigen.

Question 11

What is avidity

Answer 11

The overall strength of multiple interactions between an antibody with multiple binding sites and a complex antigen with multiple epitopes.

Question 12

What are antibody cross-reactivity and give two examples

Answer 12

Antibodies produced in response to one antigen can cross-react and bind to a different antigen of similar structure.

Vaccination for cowpox induces antigens which are able to recognise smallpox.

ABO blood group antigens - some antibodies made against microbial agents can cross-react with carbohydrates in erythrocytes e.g. in transfusions, if you give the erythrocytes to someone who has made antibodies against gut bacteria, they can cross-react with receptors on the erythrocytes and destroy them resulting in a severe reaction.

Question 13

What are the 5 classes of antibodies

Answer 13

Classes
IgG-
gamma heavy chain
3 Ch domains

IgA
alpha heavy chain
3 Ch domains

IgM
Mew heavy chain
4 Ch domains

IgD
Delta heavy chain
3 Ch domains

IgE
epsilon heavy chain
4 Ch domains

Question 14

Describe the IgG antibody

Answer 14

g heavy chain

Most ABUNDANT immunoglobulin

Monomer

FOUR subclasses: vary mainly in the hinge region and effector function domains

Moves across the placenta

Major activator of the classical complement pathway

There are variations in the hinge region

Some have lots of disulphide bonds and some don’t

Question 15

Describe the IgA antibody

Answer 15

a heavy chain

Second most abundant immunoglobulin

Blood - Monomer

Secretions - Dimer

Protects mucosal surfaces from bacteria, viruses and protozoa

Secretory IgA
They are joined together by a J chain (joining chain)
Secretory Component - helps protect the molecule from being degraded by the enzymes found on mucosal surfaces

Question 16

Formation of Secretory IgA

Answer 16

Firstly, the IgA is produced by a plasma cell

The dimeric IgA binds to the Poly-Ig Receptor on the basolateral membrane of the epithelial cell.

This triggers endocytosis

The poly-Ig receptor is cleaved which leaves the secretory component attached to the dimeric IgA

It is then secreted

THE SECRETORY COMPONENT IS DERIVED FROM THE POLY-Ig RECEPTOR

Question 17

Describe IgM antibodies

Answer 17

m heavy chain

Pentameric

5 monomers joined by the J chain - 10 Fab

Mainly confined to blood (80%)

First Ig produced after exposure to antigen

PRIMARY ANTIBODY RESPONSE

Multiple binding sites compensate for low affinity (high avidity)

Efficient at AGGLUTINATION

Activates complement

Held together by disulphide bonds

Question 18

IgD

Answer 18

d heavy chain

LOW serum concentrations

Surface IgD expressed early in B cell development

Involved in B cell development and activation

Question 19

IgE

Answer 19

e heavy chain

LOW concentration in blood

Produced in response to parasitic infections and in allergic diseases

Binds to high affinity Fc receptors of mast cells and basophils

Cross-linking by antigen triggers mast cell activation and histamine release

IgE coats mast cells and bind to the Fc receptor on mast cells

The allergen will bind to the IgE which are bound to the mast cell and cause the release of histamines.

Question 20

Where are each of the classes distributed

Answer 20

Blood = IgG + IgM

Extracellular Fluid = IgG

Secretions across epithelia (inc. breast milk) = Dimeric IgA

Foetus = IgG

Mast Cells below Epithelia = IgE

Question 21

What each antibody good at

Answer 21

IgG is good at neutralising

IgM is good at agglutinating

IgG is good at opsonisation

IgM and IgG are good at activating complement

IgE is involved with mast cell sensitisation

IgG is good at binding to virus infected cells and NK cells can recognise cells coated in IgG - Antibody Dependent Cell-mediated Cytotoxicity (ADCC)