Lecture 10 - B lymphocytes

Question 1

Identify the cell type

Answer 1

Lymphocyte

Question 2

Are T cells and B cells distinguishable microscopically?

Answer 2

No.

They are distinguished by their surface molecules.

Question 3

What are the two major types of T cell receptors (TCRs)?

Answer 3

Alpha-beta

Gamma-delta

Question 4

In which type of lymphocyte might you find a CD3 molecule?

Answer 4

T cell

CD3 acts as a signal transduction molecule for the TCR.

Question 5

Learning objective 1

List the important surface molecules that distinguish T and B cells and describe their function.

Answer 5

T cell

• T cell receptor (never secreted)
  
  • Two major categories: Alpha-beta and Gamma-delta
  • CD4 receptor on Th cells: binds constant region of MHC II
  • CD8 receptor on Tc cells: binds constant region of MHC I
• CD3 molecule
  
  • signal transduction molecule for TCR

B cell

• B cell receptor (immunoglobulin with transmembrane region)
  
  • Ig Alpha and Ig Beta act as signal transduction molecules
• Complement, Fc, and cytokine receptors.

Question 6

What does CD stand for?

Answer 6

Cluster of Differentiation

CD terminology is used to describe molecules located on the cell surface.

Question 7

What is the function of CD 11 and CD 18 molecules?

Answer 7

These are integrin adhesion molecules.

Question 8

Describe the process of flow cytometry to classify cells by immunophenotype.

Answer 8

Antibodies (example: anti-CD8) covalently bound to dyes are added to cell populations. The antibodies will bind to their specific receptor and specific cell types (example: Tc cells) will glow specific colors.

The flow cytometry machine will then count the numbers of cells glowing different colors.

Question 9

Describe the function of a fluoresence-activated cell sorter.

Answer 9

A machine that rapidly separates cells based on size or color.

Cells tagged with fluorescent antibodies can be assigned a charge and separated into different containers.

Question 10

Learning objective 2

Explain how B and T cells are identified

Answer 10

T and B cells cannot be distinguished with a light microscope. They are distinguished through which surface molecules they express.

B cells express BCRs and the transduction molecules Ig Alpha and Ig Beta.

All T cells express CD3. Depending on T cell type, they may express CD4 or CD8.

T and B cells can be identified and counted through the process of flow cytometry. They can be separated into pure populations with a fluorescence-activated cell sorter.

Question 11

Two theories exist for the generation of antibody diversity: the insructive theory and the clonal selection theory. Which is correct?

Answer 11

Clonal selection theory

B cells are not “taught” to make antibodies by being exposed to an antigen. Instead, each B cell has a specific B cell receptor. If that B cell happens to encounter its antigen, it will undergo the process of clonal expansion.

Question 12

True or False:

All of the offspring of a single B or T cell have the same antigen specificity as the original cell.

Answer 12

TRUE

When a B or T cell encounters its antigen, it will undergo mitosis to produce a number of daughter cells expressing the same antigen specificity. This process is termed clonal expansion.

Question 13

What process is being illustrated here?

Answer 13

Clonal expansion

A lymphocyte has encountered its antigen and undergone mitosis to produce a number of daughter cells. Note how all daughter cells express the same antigen-specific receptor.

Question 14

True or False:

During an infection with a specific pathogen, we will most likely see a monoclonal antibody population circulating in the plasma.

Answer 14

FALSE

Pathogens have many epitopes. Thus, several different B cell populations will undergo clonal expansion, and the resulting antibody population will be polyclonal.

This process is termed polyclonal activation

Question 15

When a B cell is activated, what 2 cell types are produced from its clonal expansion?

Answer 15

Plasma cell

Memory B cell

Question 16

What is the function of a plasma cell?

Answer 16

Produce antibodies

Question 17

What is the function of a memory B cell?

Answer 17

If an antigen is encountered a second time, memory B cells will quickly differentiate into effector cells (i.e. plasma cells) for quick secretion of antibodies.

Question 18

Describe what occurs when a B cell recognizes a self antigen during its maturation.

Answer 18

Undergoes apoptosis

This process promotes Tolerance of self antigens.

Question 19

Learning Objective 3

Explain the basic concepts of clonal selection theory and how it relates to tolerance and memory.

Answer 19

Lymphocytes carry receptors specific to a single antigen.

If a lymphocyte encounters a self antigen during development, it undergoes apoptosis. Tolerance

If a mature lymphocyte encounters its antigen, it will undergo clonal expansion into effector cells or memory cells. Memory cells are quickly activated during a secondary infection to promote a quick immune response. Memory

Question 20

Where do B cells mature?

Answer 20

In the primary lymphoid tissues

Bone marrow in mammals

Bursa of Fabricius in birds

Question 21

Which immunoglobulin serves as a maturation marker for B cells?

Answer 21

IgD

Question 22

How do B cells develop their BCRs?

Answer 22

Rearrangement of their DNA

This occurs in the primary lymphoid tissues

Question 23

Where do B cells typically encounter their antigens?

Answer 23

In the secondary lymphoid tissues

The antigen will drain into lymph nodes via the lymphatics, where a B cell can contact the antigen.

Question 24

How long does it take for a B cell to differentiate into effector cells?

Answer 24

5 days

Question 25

Why isn't IgM immediately present in the plasma after initial exposure to an antigen?

Answer 25

B cells are undergoing clonal expansion. Recall that it takes approximately 5 days for B cells to differentiate into effector cells.

Question 26

During an infection, an individual can experience enlarged lymph nodes. Why is this?

Answer 26

B and T cells are undergoing rapid clonal expansion. This process can be painful.

Question 27

During an initial infection with an antigen, some B cells will differentiate into plasma cells. Which antibody class will they secrete initially?

Answer 27

IgM

Question 28

What influences a B cell to switch antibody isotypes?

Answer 28

A mixture of cytokines released from Th cells. In this way, B cells can be influenced to release an antibody isotype more specific to the antigen in question (ex: switch from IgM to IgG).

Question 29

Why is antibody production faster in the secondary response than in the primary response?

Answer 29

In the secondary response, memory cells are already present and can differentiate into plasma cells. In the primary response, lymphocytes must undergo clonal expansion, which takes time.

Question 30

Learning Objective 4 Compare and contrast a primary and secondary response. Include the basis for the difference and lag time, titer, antibody class, and antibody affinity.

Answer 30

Primary response * Lymphocytes encounter their antigen and undergo clonal expansion. * After 5 days, B cells have differentiated into plasma cells and memory cells. Plasma cells begin releasing IgM. * Based on infection type, Th cells can influence B cells to switch the isotype of antibody being released. Secondary response (anamnestic response) * Memory lymphocytes are already present in circulation and can differentiate into effector cells more rapidly. * Some IgM is produced, similarly to primary response. * Clonal memory cells produce the specific antibody isotype from the previous infection without having to class switch. * Antibody production is higher titered and higher affinity.

Question 31

Describe the primary differences between a B cell and a plasma cell.

Answer 31

B cells are predominantly nucleus and are not very active. Will circulate for 4 months searching for its antigen. Plasma cells have a lot of RER and Golgi and are protein-exporting machines. Release about 300 antibody molecules per second. Some live 5-6 days and then die. Others are long-lived for periods of up to years.

Question 32

Define "Central tolerance"

Answer 32

If a lymphocyte encounters a self antigen during maturation, the following events may occur: B cell * Apoptosis * Rearrange DNA to edit BCR CD4+ T cell * Become a Treg cell, which suppresses T cell responses to the antigen

Question 33

Define "peripheral tolerance"

Answer 33

If a mature naive lymphocyte encounters a self antigen and does not receive danger signals, it may: * Die by apoptosis * Become anergic (inactive) * Be suppressed

Question 34

A mature naive B cell will express which two immunoglobulins on its cell surface?

Answer 34

IgM and IgD Recall that IgD is a marker of B cell maturation

Question 35

Learning Objective 5 List the way tolerance is induced - include central and peripheral tolerance.

Answer 35

Central tolerance * If an immature B cell recognizes self, it undergoes apoptosis or rearranges its BCR. * If an immature CD4+ T cell recognizes self, it becomes a Treg and suppresses T cell responses to that antigen. Peripheral tolerance * If a mature naive lymphocyte recognizes self without the presence of danger signals, it may undergo apoptosis, become anergic, or be suppressed.

Question 36

True or False: IgM is a monomer when acting as a BCR on the B cell surface.

Answer 36

TRUE

Question 37

Learning Objective 6 Understand the role of IgM and IgD in B cell development and where they are expressed.

Answer 37

1. Immature B cell rearranges its genes to develop its specific BCR in the form of an IgM molecule. 2. If IgM recognizes self in primary lymphoid tissue, cell will be deleted or will rearrange its genes again to form a new BCR. 3. B cell migrates to secondary lymphoid tissues, where it develops IgD expression. IgD is a B cell maturation marker. 4. B cell can now be activated by contacting its antigen.

Question 38

Suppose you wanted to produce a large batch of monoclonal antibodies. How would you go about doing it?

Answer 38

Inject mouse with an antigen to produce clonal expansion in the spleen. Harvest the spleen for B cells. Combine B cells with myeloma cells. This forms a "hybridoma," creating an immortal line of antibody-producing cells. Dilute cells into assays containing 1 cell apiece. Test supernatant for desired antibody. Harvest cells that are producing the desired antibody.

Question 39

Why is it necessary to create a hybridoma when harvesting monoclonal antibodies instead of just growing B cells in vitro?

Answer 39

After a few divisions, B cell populations will die off. Thus, it is necessary to combine them with immortal myeloma cell lines.

Question 40

Learning Objective 7 Define hybridoma and include in your definition the cells that are used to create hybridomas and the purpose of hybridomas

Answer 40

Hybridoma: the combination of a B cell with a myeloma cell. The B cell portion of the hybridoma produces antibody, while the myeloma (plasma tumor cell) confers immortality to the cell line. This technique allows for the production of large batches of monoclonal antibodies.