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Identify the cell type



Are T cells and B cells distinguishable microscopically?


They are distinguished by their surface molecules.


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




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

T cell

CD3 acts as a signal transduction molecule for the TCR.


Learning objective 1

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

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.


What does CD stand for?

Cluster of Differentiation

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


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

These are integrin adhesion molecules.


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

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.


Describe the function of a fluoresence-activated cell sorter.

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.


Learning objective 2


Explain how B and T cells are identified

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.


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

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.


True or False:

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


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.


What process is being illustrated here?

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.


True or False:

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


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


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

Plasma cell

Memory B cell


What is the function of a plasma cell?

Produce antibodies


What is the function of a memory B cell?

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.


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

Undergoes apoptosis

This process promotes Tolerance of self antigens.


Learning Objective 3

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

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


Where do B cells mature?

In the primary lymphoid tissues

Bone marrow in mammals

Bursa of Fabricius in birds


Which immunoglobulin serves as a maturation marker for B cells?



How do B cells develop their BCRs?

Rearrangement of their DNA

This occurs in the primary lymphoid tissues


Where do B cells typically encounter their antigens?

In the secondary lymphoid tissues

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


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

5 days


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

B cells are undergoing clonal expansion.

Recall that it takes approximately 5 days for B cells to differentiate into effector cells.


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

B and T cells are undergoing rapid clonal expansion.

This process can be painful.


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



What influences a B cell to switch antibody isotypes?

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).


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

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.


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.

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.