Lecture 2: B Cells and Generation of Antibodies

Question 1

Basic Structure of Ig

Answer 1

• Each molecule of immunoglobulin contains two identical heavy chains and two identical light chains (H2L2)
• Each chain has a constant region and a variable region
• The antigen binds to the antibody at hypervariable regions of the VH and VL
• The class of the molecule is determined by the heavy chain constant region

Question 2

Heavy Chain

Answer 2

One type of protein chain in an immunoglobulin molecule

There are several different classes of heavy chains (A,D,E,G,M) which confers a functional activity on the antibody molecule

Each immunoglobulin contains two identical heavy chains

Heavy Chains are made up of 3 constant regions and 1 variable Region

Question 3

Isotype

Answer 3

An antibody class which is determined by the constant region of the heavy chain

Heavy chains are classified as alpha, delta, epsilon, gamma, and mu

The heavy chain deterines the isotype

IgA-alpha

IgD-delta

IgE-epsilon

IgG-gamma

IgM-mu

Question 4

Light Chain

Answer 4

The smaller polypeptide chain that makes up an immunogloulin molecule

Consists of one V and one C domain

Disulfide bonded to the heavy chain

There are two isotypes of light chains known as kappa and lambda (determined by the constant region of the light chain)

Question 5

Domain

Answer 5

An Ig Domain is 110 amino acids with an intradomain disulfide bond

Question 6

V Domain

Answer 6

The amino terminal potein domain of the polypeptide chains of immunoglobulins

The most variable part of the chain

VL and VH

Question 7

C Domain

Answer 7

The part of an immunoglobulin protein chain that is relatively constant in amino acid sequence between different molecules

The constant region of the antibody determines its particular effector function

CL, CH1, CH2, CH3

Question 8

Ig Hypervariable Region

Answer 8

The complementarity determining regions of the heavy and light chain variable domain form the antibody binding site

The most variable region of the V domain

There are 3 CDR regions in each V domain

Question 9

CDR

Answer 9

Complementarity Determining Region

Most of the differences among antibodies fall within the CDR

It is the CDRs on both light and heavy chains that constitutes the antigen binding site of the antibody molecule

Question 10

Fc Region

Answer 10

The c terminal domain of the constant region of the antibody

Question 11

Fc Receptor

Answer 11

The cell surface receptor specific for the Fc portion of certain classes of immunoglobulins

Present on lymphocytes, mast cells, macrophages, and other accesory cells

Question 12

Classes of Ig

Answer 12

1. IgA
2. IgD
3. IgE
4. IgG
5. IgM

*differ in the constant region of the heavy chain

Question 13

IgA

Answer 13

The class of immunoglobulin charachterized by alpha heavy chains

Can occur in monomeric and dimeric form

Dimeric IgA is the main antibody secreted by mucosal lymphoid tissues

Major antibody in secretions such as saliva, tears, and breast milk

Plays a primary role in protection of pathogens that invade the gut or respiratory mucosal

A j chain holds the molecules together and a secretory component bridges the two IgA molecules in a process of transcytocysis from the extracellular fluid into the lumen

Question 14

IgD

Answer 14

The class of immunoglobulin charachterized by delta heavy chains.

Appears as surface immunoglobulin on mature B cells but its function is unknown

Question 15

IgE

Answer 15

The class of immunoglobulin charachterized by epsilon heavy chains.

Involved in the defense against parasite infections and allergic reactions.

IgE does not have to be agregated to bind to the Fc receptor, but once antigen binds agregation occurs

Found on basophils and mast cells which have an IgE specific receptor

Question 16

IgG

Answer 16

The class of immunoglobulin charachterized by gamma heavy chains

The most abundent class of immunoglobulin in the serum

Crosses the placenta and represents the serum antibody of newborns

IgG antibodies can bind antigen and the complex is taken up by macrophages or neutrophils through an IgG specific Fc receptor (the antigen must have an agregation of IgG on the surface to activate binding) and is then killed

Question 17

IgM

Answer 17

The class of immunoglobulin charachterized by mu heavy chains

Found as a monomer on B cells and is secreted by plasma cells as a pentamer

First immunoglobulin to appear on the surface of B cells and the first to be secreted - First serum antibody made in a primary immune response

Question 18

Transcytosis

Answer 18

IgA undergoes transcytosis where An IgA dimer binds to a polyimmunoglobulin receptor (pIgR) on the extracellular fluid side of an endothelia cell

Once bound to the Fc receptor, the IGA dimer is taken into the cell and passed into the lumen through transcytosis, leaving a secretory component from the pIgR

Question 19

Organization of Ig H and Ig L genes and rearangements to produce heavy and light chains

Answer 19

During B cell development DNA rearangment occurs and is mediated by RAG proteins

V and J segments are chosen and spliced together to form the light chain

V, J, and D segments are chosen and spliced togeteher to form the heavy chain

Question 20

B Cell Development

Answer 20

B cells develop in the bone marrow

1. Common lymphoid progenitor
2. Pro-B cell - intracellular mu chain
3. Pre-B cell - mu chain is on the surrface of the cell, and a surrogate L chain is present
4. Immature, naive B Cell- the surrogate L chain is replaced by the correct L chain

Immature B cells leace the bone marrow and develop into mature naive B-cells which are waiting for the antigen while circulating through the periphrial lymphoid organs

Question 21

Affinity Maturation

Answer 21

Following the introduction of an antigen, the VDJ heavy chain and the VJ light chain undergo a somatic hypermutation and high affinity antiboies are selected

This requires the enzyme AID - activation induced cytidine deaminase

Upon initial exposure to the antigen, antibody binding does not have a high affinity, however through the process of somatic hypermutation the Kd value devreases and infinity increases.

Somatic hypermutation occurs most often at the antigen binding site (CDR1, CDR2, and CDR3)

Question 22

Antibody diversity generation

Answer 22

A large antibody repritoire is developed through gene rearrangments and somatic hypermutation

Question 23

Combinatoral Joining

Answer 23

A large heavy chain repertoire develops by combinatorially joining any V gene segment with any D gene segment with any J gene segment

A large light chain repertoire develops by combinatorially joining any V gene segment with any J gene segment

The combination of any heavy chain with any light chain results in a large antibody repertoire

Question 24

Allelic Exclusion

Answer 24

Only one of the two IgH and IgL aleles is rearranged functionally so that one B cell makes only a single H and L chain

Question 25

VDJ gene rearangment

Answer 25

DNA gene rearangments occur during B cell development

In the common lymphoid progenitor, one D segment is selected to join with one J segment on the heavy chain

The DJ segment then joins with one V segment and the heavy chain migrates to the surface of the Pre-B cell where is is joined by surogate light chains. The surrogate light chains then send a signal back to the cell to stop rearrangment of the heavy chain

The immature B cell still in the bone marrow then undergoes VJ rearangment of either the kappa or lambda light chain which replaces the surrogate light chain sending a signal back to the cell to stop rearrangment

*DNA cleavage is mediated by RAG enzyme

Question 26

Defficency in RAG enzyme

Answer 26

A defficincy in the RAG enzyme would prevent B cells and T cells from forminh and would result in an immune defficent animal.

Question 27

Isotype switching

Answer 27

Durring the secondary immune response IgM bearing B cells switch to produce another isotype such as IgG, IgE, or IgA

This occurs through DNA deletion by cutting and rejoining of DNA near switch sequences

Isotype switching is not RAG dependent, it requires the enzyme AID