B cell activation Flashcards
What is the whole point of B cell activation?
To get the B cell to synthesize and secrete antibodies (the secreted form of their antigen receptor).
B cells can respond to antigen in a T cell-independent (absence of T cell help) or T cell-dependent (involves T helper cells) manner depending on the chemical nature of the antigen.
How will B cells respond to antigen?
B cells can respond to antigen in a T cell-independent (absence of T cell help) or T cell-dependent (involves T helper cells) manner depending on the chemical nature of the antigen.
Describe T-independent antibody responses.
Usually antigens with repeating determinants such as polysaccharides that can cross-ink multiple B cell receptors (BCRs) on the B cell.
The B cells that are activated secrete low-affinity IgM antibodies.
The antibody response is relatively fast to develop, but the antibodies are usually low-affinity IgM antibodies.
Memory responses do not develop with T-independent antigens.
What type of immunoglobulins do B cells secrete when engaged in a T-independent antibody response?
Low-affinity IgM antibodies.
Describe the speed of the T-independent antibody response.
Relatively fast to develop, but the antibodies are usually low-affinity IgM antibodies.
Do memory responses develop through T-independent antigens?
No.
What does B cell activation to T cell-independent antigens require?
Extensive BCR cross-linking
What are the two signals for B cell activation to T cell-dependent antigens?
BCR binding to antigen and co-stimulation through CD40:CD40L on T helper cell.
This type of antibody response takes longer to develop.
Some B cells differentiate to plasma B cells and secrete antibodies.
The functions of antibodies include neutralization, opsonization, and complement activation.
Other B cells differentiate to memory B cells. Class-switching to other Ig isotypes may occur.
What are some functions of antibodies secreted by B cell activation to T-cell dependent antigens?
neutralization, opsonization, and complement activation.
What are T-dependent antibodies made in response to?
Usually to protein antigens.
These antigens usually do not have repeating determinants that can cross-link large numbers of BCRs together.
What is the key feature of T-dependent antibody responses?
Class switching and production of long-lived plasma cells and memory cells.
The antibody response is relatively slow to develop.
In time, the B cells that produce antibodies that bind to antigens with increased affinity are selected over those that produce antibodies that bind with lower affinity.
The activation of mature, naïve B cells to T-dependent antigens normally requires two signals.
What is the first signal?
What provides the signal?
Provided when antigen binds to two adjacent BCRs.
The cross-linking activates a signaling cascade initiated by the Igα/Igβ co-stimulatory proteins of the BCR.
The B cell internalizes the antigen by receptor-mediated endocytosis.
The antigen is then processed inside an endosome and peptide fragments are displayed on MHC class II proteins.
This signaling cascade initiated by the Igα/Igβ also up-regulates MHC class II proteins and B7 proteins on the B cell surface, enabling the B cell to act as an antigen-presenting cell for a previously activated T helper cell. When a T helper cell has been activated by a dendritic cell, it expresses a cell surface molecule called CD40L.
The T cell receptor (TCR) of the T helper cell then recognizes the antigenic peptide displayed on the MHC class II protein of a B cell, and the CD28 of the T helper cell binds to the B7 of the B cell.
What is the second signal of B cell activation?
What delivers the signal?
The second signal of B cell activation is delivered when the CD40 protein on the B cell binds to the CD40L of the T helper cell.
The B cell also receives cytokines from the T helper cell, resulting in the complete activation of the B cell.
The B cell is then able to proliferate and differentiate into antibody-secreting plasma cells.
What results in complete activation of the B cell?
The B cell also receives cytokines from the T helper cell, resulting in the complete activation of the B cell.
The B cell is then able to proliferate and differentiate into antibody-secreting plasma cells.
What does the up-regulation of MHC class II proteins and B7 proteins on the B cell surface enable the B cell to do?
Act as an antigen-presenting cell for a previously activated T helper cell.
When a T helper cell has been activated by a dendritic cell, it expresses a cell surface molecule called CD40L.
The T cell receptor (TCR) of the T helper cell then recognizes the antigenic peptide displayed on the MHC class II protein of a B cell, and the CD28 of the T helper cell binds to the B7 of the B cell.
The V regions of the heavy and light chains are involved in antigen recognition.
True or false?
True.
Cell surface immunoglobulins associate with the signalling proteins Igα and Igβ.
True or false?
True.
B cells react with antigens through T cell receptor antigen receptors.
True or false?
False.
The B cell receptor is made of one copy of the heavy chain and one copy of the light chain.
True or false?
False.
The second signal for B cell activation is the interaction of B7 on the B cell with CD28 on the T helper cell.
True or false?
False
The CD40 ligand (CD40L) is expressed on B cells after the binding of antigen, and on macrophages after they have engulfed a bacterial cell.
True or false?
True.
After binding an antigen, the B cell internalizes it into an endosome.
True or false?
True.
If the antigen contained protein, the B cell may display a peptide fragment on its MHC class I proteins.
True or false?
False
B cells can act as antigen presenting cells in order to receive “signal 2” from the T helper cell.
True or false?
True.
If an antigen is made of several repeating units, it might be able to activate a B cell without any help from a T cell.
True or false?
True.
T cell help is needed for the B cell to generate high affinity class switched antibodies.
True or false?
True
T dependent antigens usually do not have a lot of repeating subunits.
True or false?
True.
B cells express the CD40L protein after they have recognized an antigen and received “signal 1.”
True or false?
False
