Antibody Structure and Function

Question 1

Antibodies

Answer 1

Immunoglobulins
Glycoproteins in the surface of B cells and secreted by plasma cells
Present in blood serum and secreted fluids such as saliva and milk
Activate the classical complement pathway
Serve as opsonins to enhance phagocytosis
Neutralize toxins and viruses
Function as antigen receptors for B cells

Question 2

Immunoglobulin Structure

Answer 2

Monomers (A, D, E, G, M): 2 identical heavy chains and 2 identical light chains held by disulphide bonds and non-covalent interactions
Globular domains of 110aa formed by intrachain DS bonds
V region: variable aa
C region: constant aa
Glycosylation: increases stability, affects protein interactions
J chains only found in IgM and IgA

Question 3

Light chains

Answer 3

(kappa, lambda)
kappa is expressed first unless something is wrong
1VL and 1CL domain

Question 4

Heavy chains

Answer 4

(alpha, sigma, epsilon, gamma, nano)
1VH domain and 3 or 4 constant domains designated by CH1-4
Difference in various heavy chain constant region domains determines antibody half-life, distribution, complementation fixing ability and Fc receptor binding

Question 5

V region

Answer 5

3 hypervariable regions within the V region show greater aa variability and form antigen-binding site (amino terminal)

Question 6

Hinge region

Answer 6

Proline-rich aa sequences between CH1 and CH2
IgA, IgD, IgG
CH2 region of IgM and IgE are flexible

Question 7

C region

Answer 7

Carboxyl terminal of heavy chain

Question 8

Proteolysis of IgG

Answer 8

Certain enzymes can cleave IgG into specific fragments

Question 9

Proteolysis of IgG by papain

Answer 9

Yields 2 Fab fragments (fragment antigen binding) and 1 Fc fragment (fragment crystallizable)

Question 10

Proteolysis of IgG by pepsin

Answer 10

Yields divalent F(ab)2 and pFc

Question 11

Isotopic determinants

Answer 11

Located in constant regions and define heavy chain classes/subclasses and light chain types/subtypes within a species

Question 12

Allotropic determinants

Answer 12

Located in constant regions and very between individual to individual

Question 13

Idiotypic determinants

Answer 13

Located within heavy and light chain variable regions and defined by unique aa sequences that determine antibody specificity

Question 14

Opsonization

Answer 14

Interactions with Fc receptors on phagocytes promotes phagocytosis

Question 15

Complement activation

Answer 15

Responsible for inactivation/removal/killing of pathogens

Question 16

Antibody-depending cell-mediated cytotoxicity

Answer 16

ADCC

Antibody acts as a receptor to enable recognition and killing of target cells by Fc receptor bearing NK cells

Question 17

Transcytosis

Answer 17

Passage across epithelial layers delivers certain classes of antibody (primary IgA) to mucosal surfaces

Question 18

Induction of mast cell degranulation

Answer 18

IgE only

Question 19

IgG

Answer 19

Consists of 2 50kDA gamma and 2 25kDA kappa/lambda chains
4 subclasses in humans
Most abundant immunoglobulin in serum and extravascular spaces
Neutralizes viruses/toxins, opsonizes microbes, activates classical complement pathway, medics ADCC reactions, crosses the placenta during pregnancy

Question 20

IgM

Answer 20

Monomer, 2 nano chains, 2 kappa or lambda chains, on B cells
Secreted into plasma cells as pentamer with J chain
Good at capturing antigens
only 5/10 antigen binding sited on pentamer are able to physically interact with large antigens (steric hindrance)
First Ab response in newborns (in response to cytokines, other Ab are made)
Efficient agglutinator of particulate antigens
Activator of complement pathway
Low concentration
Transport across epithelial mucosa

Question 21

IgA

Answer 21

Present as a monomer (2 alpha, 2 kappa or lambda) in serum and as a dimer in mucosal secretions (J chain and secretory component)
Dimeric IgA is formed in plasma cells by addition of J chain, secretory component is added to dimer during its passage through glandular epithelial cells to protect the dimer from proteolysis
IgA-antigen complexes are trapped in mucus and eliminated by mechanical action
In breast milk

Question 22

IgE

Answer 22

Level is low in serum except in allergy or parasitic infections
Binds high affinity Fcepsilon receptors on mast cells and basophils
Cross linking membrane-bound IgE cause mast cell and basophil degranulation, leading to acute inflammation as well as allergic responses
Responsbile for the symptoms of atopic allergy
Important role in host defence against certain parasitic worms

Question 23

IgD

Answer 23

Exists as a monomer (2 sigma chains, 2 kappa or lambda chains)
Level is very low in serum - prone to proteolysis
Mostly on surface of B cells
Involved in B cell activation

Question 24

Antigen-binding receptor on B cells

Answer 24

Membrane bound immunoglobulin (mIg), very short cytoplasmic tails and cannot interact with intracellular signalling molecules
Disulphide-linked heterodimer Ig-alpha + Ig-beta
Cytoplasmic tails of Ig-alpha and beta interact with intracellular signalling molecules such as tyrosine kinases

Question 25

Immunoglobulin Superfamily

Answer 25

Molecules associated with immune function exhibiting Ig folding domain structure
Include Ab, Ig-alpha, Ig-beta, T cell receptor, Fc receptor, CD4, major histocompatibility complex molecules and various CAMs
Thought to have originated through mutation and duplication from a single primordial gene encoding a polypeptide of 110 aa

Question 26

Monoclonal antibodies

Answer 26

Derived form single B cell clone and specific to single epitope
Produced by immortal hybridoma cells created by fusing antigen-specific B cells with a transformed (cancerous) myeloma cell
Grown on special medium
Diagnostic, imaging, therapeutic
Culture or in vivo