B cell immunity helps destroy __________ pathogens.
Extracellular
_________ bind to microbial agents and help in their destructions.
Antibodies
This cell type produces antibodies.
B cells
________ go back to infected tissues, bind to pathogens and coat their surface.
Antibodies
BCR can bind directly to what antigen type(s)?
Protein antigens and non-protein antigens
TCR can only bind to what type of antigen?
Protein antigen
Primary response (in B cells) produces these cell types.
Effector cells and memory cells
Where does the naive B cell encounter its specific epitopal region?
While circulating through the lymphoid tissues.
What is the function of plasma cells?
Secrete antibodies
What is a difficulty when studying B cells?
Naive B cells and memory B cells look identical (can only differentiate with surface markers)
What is the BCR?
membrane bound Ig molecules
What are antibodies?
Soluble Ig molecules
Can more than one B cell bind to a specific antigen?
Yes, multiple B cells can bind to different epitopes on the same antigen.
B cell circulates through ________ ________ ________. If a B cell doesn not bind its antigen, what happens?
Secondary lymphoid tissues
it remains inactive and re-circulates. (Ultimately will die of apoptosis if it doesn’t meet its antigen)
What are the necessary requirements for a B cell to undergo clonal expansion?
Binding to its specific antigen and a Tfh cell
Antibodies return where?
What do they help fight against?
Return to infected tissue.
Help fight EXTRAcellular pathogens.
How many Ig isotypes are there? What are they?
5
IgA, IgM, IgD, IgG, IgE
What is the differentiating factor between Ig classes?
NOT the variable region, but rather differences in the heavy chain constant region.
Where do all immune cells arise from?
Bone marrow
B cells, after development, arrive in the blood under what form?
(Mature) naive B cells
What Ig receptor(s) do naive B cells express?
Co-express IgM and IgD
Do the co-expressed IgD and IgM have different specificities?
No, only differences in heavy chain constant region. Bind to the same epitope (or specific for more accurately)
Which B cell type is the one that circulates through the SLT and responds to pathogens it has specificity for?
Naive B cell
The most common B cell response is against what type of antigens?
Protein antigens
B cell response to a protein antigen requires the help of what cell type?
Tfh cell
Describe the events starting in the SLT, to binding to a Tfh cell.
Naive B cell circulates in the SLT and binds to protein antigen. B cell takes up pathogen by endocytosis - lysosome fuses with endosome. B cell presents peptide fragments on MHC class 2 molecule to a Tfh cell with the same specificity.
What happens after B and Tfh cell binding?
Both the B and T cell get activated, leading to clonal expansion for both.
This binding causes release of Tfh cell cytokines.
What are the three fates of a naive B cell undergoing clonal expansion?
1 - Memory B cells - co-express IgM and IgD
2 - Short-lived plasma cells - secrete IgM antibodies
3 - Tfh cells and other B daughter cells form a germinal center, where the GC B cells undergo class switching
What is class switching?
GC B cell switches its Ig class (from IgD/M –> IgG/A/E)
How is class switching accomplished (Genetically)?
Re-arrangement of the heavy chain constant region genes.
Does class switching affect antigen binding?
No, variable region is unaffected - so antigen specificity remains the same.
What is the Ig class change (during class switching) determined by?
Tfh cytokines
What are the two possible fates of a GC B cell?
Either becomes a long-lived plasma cell (secretes antibody type it class switched to) or becomes a memory B cell (Expresses Ig molecule as BCR it class switched to)
What antibody type is always secreted first?
IgM
Where does class switching occur?
Germinal center
Why is class switching even necessary?
IgM doesnt enter the tissues
Where do long-lived plasma cells reside? What do they do?
Reside in the bone marrow
Constitutively secrete antibody
How long do long-lived plasma cells reside in the bone marrow?
Months to years
How long do short-lived plasma cells live for?
5-10 days
Where is adaptive immunity always initiated?
SLTs
Describe, how Tfh and B cells get activated, and their outcomes.
Dendritic cell phagocytoses extracellular bacterium. Moves to the secondary lymphoid tissues and presents peptide on MHC class 2 molecule to naive Th cell, which becomes a Tfh cell.
Next, pathogen in SLTs, taken up by naive B cell by endocytosis, presents peptide on MHC2 molecule and binds to Tfh cell.
Naive B cell and Tfh cell undergo clonal expansion. B cell can become a memory cell, plasma cell or a GC B cell. Within the germinal center, Tfh cell releases cytokines, which dictate to what class the B cell can switch. The GC B cell can either become a memory cell or long-lived plasma cell.
In the primary response, which antibody is secreted first? By which cell?
IgM - short-lived plasma cell