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.
