B Cell Immunity

Question 1

Where do B cells develop? Where do they migrate after development?

Answer 1

B cells develop in bone marrow and then migrate into the periphery and through the secondary lymphoid tissues.

Question 2

What are the 3 kinds of B cells?

Answer 2

B-1, B-2 and Marginal Zone (MZ)

Question 3

Describe the following as relates to B-1 Cells:

When first produced-
N-Regions in VDJ junctions-
V-region repertoire-
Primary location-
Mode of renewal-
Spontaneous production of immunoglobulin-
Isotopes secreted-
Response to carbohydrate antigen-
Response to protein antigen-
Requirement for T-cell help-
Somatic hypermutation-
Memory development-

Answer 3

When first produced- fetus
N-Regions in VDJ junctions- few
V-region repertoire- restricted
Primary location- body cavities (peritoneal, pleural)
Mode of renewal- self-renewing
Spontaneous production of immunoglobulin- high
Isotopes secreted- IgM more than IgG
Response to carbohydrate antigen- Yes
Response to protein antigen- Maybe 
Requirement for T-cell help- No
Somatic hypermutation- Low to none
Memory development-  Little or none

Question 4

Describe the following as relates to Conventional B-2 Cells:

When first produced-
N-Regions in VDJ junctions-
V-region repertoire-
Primary location-
Mode of renewal-
Spontaneous production of immunoglobulin-
Isotopes secreted-
Response to carbohydrate antigen-
Response to protein antigen-
Requirement for T-cell help-
Somatic hypermutation-
Memory development-

Answer 4

When first produced- after birth
N-Regions in VDJ junctions- extensive
V-region repertoire- diverse 
Primary location- secondary lymphoid organs
Mode of renewal- replaced from bone marrow 
Spontaneous production of immunoglobulin- low
Isotopes secreted- IgG more than IgM 
Response to carbohydrate antigen- Maybe
Response to protein antigen- Yes
Requirement for T-cell help- Yes 
Somatic hypermutation- High
Memory development- Yes

Question 5

Describe the following as relates to Marginal Zone B Cells:

When first produced-
N-Regions in VDJ junctions-
V-region repertoire-
Primary location-
Mode of renewal-
Spontaneous production of immunoglobulin-
Isotopes secreted-
Response to carbohydrate antigen-
Response to protein antigen-
Requirement for T-cell help-
Somatic hypermutation-
Memory development-

Answer 5

When first produced- after birth 
N-Regions in VDJ junctions- Yes 
V-region repertoire- Partially restricted
Primary location- spleen
Mode of renewal- long lived
Spontaneous production of immunoglobulin- low
Isotopes secreted- IgM more than IgG 
Response to carbohydrate antigen- Yes 
Response to protein antigen- Yes
Requirement for T-cell help- Sometimes
Somatic hypermutation-? 
Memory development- ?

Question 6

Which B cells are innate like? Which B cell mediates most adaptive immune responses?

Answer 6

B-1 and MZ B cells are innate-like. Most adaptive immune responses are mediated by B-2 cells

Question 7

Describe what happens upon stimulation of B-2 cells.

What does magnitude and type of response depend upon?

Answer 7

Upon stimulation, B-2 cells can further differentiate into antibody-secreting plasma cells.

The magnitude and type of response depends of route of immunization: subcutaneous; intradermal; intramuscular; intravenous; intramucosal.(Figure A.2).

Question 8

What do protein antigens usually require?

Answer 8

Protein antigens usually require adjuvant, a substance that enhances immunogenicity- for human use, the adjuvant Alum,
makes the immunogen particulate and readily ingested by antigen-presenting cells (Fig A.4).

Too little immunogen results in no immune response (Figure A1)

Question 9

Describe how size, route, similarity to self protein, and adjuvants affect immunogenicity.

Answer 9

Increased immunogenicity:

• large size
• subcutaneous > intraperitoneal > intravenous or intragastric
• multiple differences from self protein
• slow release

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Question 10

Describe B cell activation: T dependent responses.

Answer 10

Most B cell responses require T cell help for activation.

The BCR interacts with antigen; the antigen is internalized with the BCR and the antigen is degraded and peptides associate with MHC
class II molecules and go to the surface of the B cell. The TCR of the T cell recognizes the peptide in the context of MHC class II molecules and is stimulated to produce cytokines which in turn activate
the B cell to proliferate and differentiate into antibody-producing cells (plasma cell

Question 11

Activation of the B cell requires two signals for T dependent and T independent responses. Describe.

Answer 11

For T-dependent responses, these two signals are:

1) interaction of BCR with antigen;
2) interaction of TCR with peptide/MHC complex and interaction between costimulatory molecules CD40 (B cell) and CD40L (T cell).

For T-independent antigens such as polysaccharides, the second signal can be provided by the antigen- no T cell is required.

Question 12

What happens once the TH cell is activated by interaction of TCR with peptide/MHC II complex?

Answer 12

Activation of TH cells by interaction of TCR with peptide/MHC II complex triggers T cells to
secrete B-cell stimulatory cytokines IL-4, IL-5 and IL-6 and to express CD40L. These cytokines plus the interaction of CD40L on T cells with CD40 on B cells drives B cells into proliferation.

Question 13

What will drive the B cell to proliferate?

Answer 13

Activation of TH cells by interaction of TCR with peptide/MHC II complex triggers T cells to
secrete B-cell stimulatory cytokines IL-4, IL-5 and IL-6 and to express CD40L. These cytokines plus the interaction of CD40L on T cells with CD40 on B cells drives B cells into proliferation.

Question 14

B and T cells must recognize the same antigen. Explain this in the context of Haemophilus influenza type B.

Answer 14

For T-dependent antigens, the B and T cells must recognize the same antigen, although they don’t recognize the same epitope.

Consider the vaccine to Haemophilus influenza type B: protective antibody is to the polysaccharides but infants do not make effective T-independent response to
polysaccharides. To circumvent this, the vaccine uses the H. influenza polysaccharide chemically
linked to a protein (tetanus toxoid) (T-dependent response). Remember that B cells develop in the
bone marrow in an antigen independent manner and then migrate to the periphery.

B cells with specificity for the polysaccharide, will bind the H. influenza polysaccharide linked to tetanus toxoid and will present peptides of the tetanus toxoid to the T cells. T cells can then interact with
and stimulate the H. influenza polysaccharide-specific B cells.

Question 15

Describe the difference between primary and secondary antibody responses.

Answer 15

Primary response to antigen is slow and is mostly IgM;

secondary responses are rapid and mostly IgG;

affinity increases as a result of
somatic hypermutation at end of primary response.

Further, the antibody titer is higher in the

Question 16

What is the major Ig of the primary response? Secondary response?

Describe isotype switching. How does it occur? When does it occur?

Answer 16

While IgM is the major Ig of the primary response, IgG (and to a lesser extend, IgA and IgE) is the major Ig of a secondary immune response.

This phenomenon is called isotype switching and occurs by rearrangement of the CH gene. Isotype switch occurs during T-dependent responses and is regulated by T-cell cytokines:

Question 17

IL-4 will induce what switch? TGF-beta? (mouse)

IFN-gamma? (mouse)

Answer 17

For example, IL-4 induces switch to IgG1 and IgE;

TGF-beta induces IgG2b (mouse) and IgA;

IFN-gamma induces IgG3 and IgG2a (mouse).

Question 18

Describe Somatic recombination (switch recombination).

What enzyme is required?

Answer 18

DNA loops out; intervening DNA is excised and is deleted from the genomic DNA. Switch recombination allows a cell to switch from synthesis of IgM to synthesis of IgG, IgA or IgE. This recombination requires the enzyme activation-induced cytidine deaminase (AID). The mechanism for switching to IgA and IgG is similar to that shown below for IgE

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Question 19

What are switch sites?

Answer 19

Switch sites = upstream repeat sequences with high degree of similarity between the different switch regions. Switch sites and AID mediate the recombination.

Question 20

Describe Mechanisms of heavy chain class (isotype) switching. Heavy chain class (isotype) switching.

Answer 20

Deletion of CH genes, of which only Cmu and Cdelta are shown, leads to recombination of the VDJ complexes with a 3’ CH gene and expression of this gene (Cepsilon shown in the example). Switch regions are indicated by dark circles. (Note that the Cgamma genes are located between Cdelta and Cepsilon but are not shown.)

Question 21

What is the significance of germinal centers? When do they form? What takes place here?

Answer 21

Germinal centers are sites of B cell proliferation and differentiation.

Germinal centers form during immune responses and here, isotype switch, somatic hypermutation of Ig genes and development of memory B cells/plasmablasts occur.

Question 22

How is generation of antibody diversity established in secondary lymphoid tissues?

Answer 22

Somatic hypermutation: Changes in antigen-binding specificity as a result of
somatic mutations in V genes can generate additional diversity.

Question 23

What enzyme is required for somatic hypermutation? What happens if a patient lacks this enzyme?

Answer 23

The enzyme, AID, is required for somatic hypermutation. AID deficient patients have only
IgM and the Ig genes are not somatically diversified.

Question 24

What happens after hypermutation?

Answer 24

After hypermutation, cells with high affinity receptors for foreign antigen are rescued from cell death by
antigen in the form of antigen-antibody complexes bound to follicular dendritic cells followed by B cell
presentation of the antigen to TH cells.

Question 25

Where do primary B cells enter? How do they leave?

Answer 25

Primary B cell blasts enter the germinal center; cells leave as memory B cells or as antibody
forming cells (plasma blasts).

Question 26

What is the maturation and differentiation of B cells and switch from IgM to IgG dependent on?

What is specifically required?

Answer 26

The maturation and differentiation of B cells and the switch from IgM to IgG is dependent on T cells

The T cells required for these processes are T helper lymphocytes (TH). TH cells enhance antibody production by two means.

Question 27

The T cells required for the maturation and differentiation of B cells and the switch from IgM to IgG are T helper lymphocytes (TH). TH cells enhance antibody production by two means. Describe.

Answer 27

Precursor TH and APC = activated TH cells

• The APC takes-up the antigen, processes it and expresses peptides in the context of MHC II molecules;
• The TCR recognizes peptide/MHC II complex and becomes activated

Activated TH cell and Ag-B cell leads to activation of B cells which will lead to plasma cells and memory cells:
- The TCR recognizes peptide/MHC II complex on B cell and the TH cells secrete cytokines which stimulate the B cell to proliferate and differentiate.

Question 28

Describe B cell activation: the T-independent response.

What antibodies are usually formed? Are they somatically mutated? What are the implications for this? What are they important in the response to?

Answer 28

With T-independent antigens, the antibodies formed are usually IgM and are not somatically mutated. Consequently, the antibodies are lower affinity than those of T-dependent antigens. They are however, important for antibody responses to bacterial polysaccharide antigens.

Question 29

What do T-independent antigens include?

What are they associated with? What does a T-independent response allow for?

What are the antigens comprised of? Are memory cells produced?

Answer 29

T-independent antigens include: Bacterial lipoppolysaccharide, polymerized flagellin, pneumococcal polysaccharide, natural polysaccharides, dextrans, levans, and hyaluronic acid. Many of these antigens are associated with bacterial products and a T-independent response likely allows a rapid response to bacterial infection. These antigens are comprised of repetitive units and B cells may be triggered by these antigens by cross-linking the BCR. These antigens do not generally promote production of memory cells.

Question 30

Which of the following is characteristic of an immune response to protein antigens:
A. The secondary response is characterized by high levels of IgM antibody
B. The primary response occurs approximately 14 days after immunization
C. Memory B cells are not expected to develop
D. Germinal centers will develop 2 weeks after secondary immunization
E. The B and T cells will recognize different epitopes of the protein

Answer 30

E