Lecture 6 - Antibody Structure and Function

Question 1

B Cells

Answer 1

1. B cells arise from bone marrow stem cells
2. Mature B cells exit bone marrow and enter circulation.
3. B cells continuously circulate in search of antigen.

Question 2

Functions of Antibodies

Answer 2

— Neutralization of microbes and tozins
— Opsonization and phagocytosis of microbes
— Antibody-dependent cellular cytotoxcicity
— Lysis of microbes
— Phagocytosis of microbes opsonized with complement fragments (eg. C3b)
— Inflammation
— Complement activation

Question 3

Antigen-Antibody/BCR Interactions

Answer 3

1. BCR detect three-dimensional antigens.
2. B cells are active only against extracellular antigens

Question 4

BCR/TCR Activation

Answer 4

1. Antigen receptors associate with cellular signaling proteins in the BCR or TCR complex.
2. When adjacent receptors bind two or more antigens the receptors are pulled together into an aggregate.
3. Cross-linking brings signaling proteins together and initiates signal transduction.
4. As the signaling proteins are the same in each clone, the transduced signal is also the same

Question 5

Clonally distributed antigen receptors

Answer 5

Each clone has a unique receptor distinct from all other receptors

Question 6

Clone

Answer 6

A parental lyphocyte and all its progeny

Question 7

B cell antibody production

Answer 7

1. Each B cell encodes its own unique BCR with its own unique antigen specificity. The antigen receptor is membrane bound on a naïve B cell.
2. Antigen plus second signal stimulate the B cell and initiate antibody production.

The BCR and corresponding antibody share IDENTICAL antigen specificities

Question 8

Antibody (Ab)

Answer 8

Aka immunoglobulin (Ig)

A type of glycoprotein produced by B lymphocytes

Antibodies bind antigen with a high degree of specificity and affinity.

Antibodies recognize a variety of three-dimensional shapes (amino acids, lipids, carbohydrates, etc)

Question 9

Ig

Answer 9

Immunoglobulin (aka antibodies)

Question 10

Antibody Structure

Answer 10

Four polypeptides:
- two light chains and two identical heavy chains form a Y-shaped molecule.
- two heavy chains connected to the light chains by a disulfide bond. The two heavy chains are connected to each other by two disulfide bonds

The tips of the light and heavy chains are the variable region (functional domain: antigen binding)
The rest is the constant region (bilogical activity)

Question 11

Antibody Light Chain

Answer 11

Contains one V domain and one C domain

Question 12

Antibody Heavy Chain

Answer 12

Contains one V domain and at least three C domains

Question 13

Antibody functional domain

Answer 13

1. The variable (V) region varies between clones and is involved in antigen recognition.
2. The constant (C) region is conserved among clones and is required for structural integrity and effector functions

Question 14

Epitope

Answer 14

The parts of an antigen recognized by an antibody are called epitopes. Epitopes can be recognized on the basis of sequence or shape

Question 15

Affinity

Answer 15

The strength with which one antigen binding surface of an antibody binds an antigen

Question 16

Antibody-mediated Antigen Recognition

Answer 16

1. Antibodies recognize a large array of 3D structures.
2. Each clone is specific for a single antigen.
3. Antigen recognition is mediated by specific domains of the antibody.
4. Signaling triggers B lymphocyte activation

Question 17

Antibody (immunoglobulin) isotypes

Answer 17

IgG, IgE, IgD, IgM, IgA

They differ in their C region and their physical/biological properties and effector functions

Question 18

IgA

Answer 18

Antibody isotype involved in mucosal immunity and neonatal passive immunity

Diamer: consists of two IgA molecules joined by a J chain.

The J chain facilitates transport of IgA across mucosal epithelia and facilitates transfer of IgA to newborns to confer neonatal passive immunity

Question 19

IgD

Answer 19

Antibody isotype

Found on naïve B cells; acts as a marker for B cell development

Function poorly understood; restricted to membrane and barely expressed on active B lymphocytes and IgD knock-out mice do not have any major defects.

Question 20

IgE

Answer 20

Antibody isotype involved in mast cell activation (immediate hypersensitivity)

Secreted as a monomer
Binds the Fcε receptor on mast/basophilic cells

Functions:
— Binds allergens and facilitates degranulation
— Facilitates protection against parasites
— Possible role in cancer immunity

Question 21

IgG

Answer 21

Antibody isotype involved in opsonization, complement activation, antibody-dependent cell-mediated cytotoxicity, neonatal immunity, and feedback inhibition of B cells

Secreted as a monomer
Most abundant antibody in bodily fluids
Can transfer to fetus

Functions:
— Neutralization of toxins
— Opsonization for inducing phagocytosis via complement system
— Facilitates antibody-dependent cytotoxicity on NK cells

Question 22

Opsonization

Answer 22

Ig binding promotes microbe internalization

Question 23

Presentation process for IgG

Answer 23

1. Opsonization: Ig binding promotes microbe internalization
2. IgG-Ag binds the Fcγ receptor of macrophages.
3. Ag is internalized through phagocytosis.
4. Microbe is degraded in the phagolysosome.
5. Microbial peptides are presented on Class II MHC molecules CD4 T cell activation

Question 24

ADCC

Answer 24

Antibody-Dependent Cellular Cytotoxicity

Mediated by Fc receptors

Question 25

ADCC mediation by Fc receptors with IgG

Answer 25

1. IgG binds Ag on surface of target cell
2. Fcγ Receptors on Natural Killer cells bind Fc of Ig
3. Cross-linking of Fc receptors signals to the Natural Killer cell to kill the target.
4. Target cell dies by apoptosis

Question 26

IgM

Answer 26

Antibody Isotype thats involved with naive B cell antigen receptors and complement activation

Pentamer with 10 different Ag binding sites

Has a J chain for secretion

IgM is the first antibody expressed in mature B cells.
Important for activation of the complement pathway (neutralization, phagocytosis)
Appears early after infection and usually not seen upon re-infection

Question 27

Affinity

Answer 27

strength of binding of antigen by a single Ab binding site

Question 28

Avidity

Answer 28

The combined strength of multiple binding site interactions an Ab can make with antigen. IgM has greater avidity than IgD as it can bind MORE epitopes at once than IgD can