B Cells 2 Flashcards
do the 2 signals of B cell activation occur at the same time or sequentially? why?
they occur sequentially
because p:BCR allows internalization of peptide for presentation of MHCII which can then interact with TCR
where does B cell activation, differentiation, and proliferation occur?
in different parts of the lymph node
what is the subcapsular sinus?
casing of the lymph node
what occurs in the subcapsular sinus?
SCS macrophages are located here and encounter Ag
what occurs in the T cell zone?
T cells reside here once they enter thru HEV and get activated by interacting with DCs/APCs
what occurs in the B cell zone?
B cells encounter Ag and undergo proliferation and differentiation
why is the B cell zone closer to the subcapsular sinus than the T cell zone?
B cells have to be in close contact to SCS macrophages to be activated so the B cell zone must be closer to the subcapsular sinus
what is the T-B border? what occurs here?
Border btwn T and B cell zones
this is where B cells receive signal 2 via T cells
where are follicles and the germinal centers located?
B cell zone
in general, what occurs at follicles?
B cells develop and get activated
in general, what occurs at germinal centers?
site of INTENSE B cell proliferation and differentiation
describe 6 steps of B cell activation
- B cell encounters free-floating Ag, Ag on SCS macrophages, or Ag on follicular DCs in the B CELL ZONE
- BCR:Ag endocytosed and processed
- increased MHC II expression
- increased chemokine receptor expression
- B cell targeted to T-B border by chemokines
- B cell receives signal 2 at T-B border and becomes activated
when does MHC II expression increase? why?
once BCR:Ag is endocytosed and processed, MHC II expression increases
lets you present more peptides to increase the chance of meeting TCR
what type of T cell is responsible for activating B cells?
Tfh
on which cells is there an increased expression of chemokine receptors? why?
T and B cells –> so they can reach the T-B border
what are the 2 choices of an activated B cell?
- form PRIMARY FOCUS in subcapsular region
- migrate to follicle to form GERMINAL CENTER
describe the 3 steps of forming a primary focus
- SIGNAL 1 –> in B cell zone
- SIGNAL 2 –> at T-B border
- B cells become plasmablasts and form primary focus near the subcapsular region
what type of cell do B cells become when they form the primary focus?
plasmablasts
describe the steps of forming a germinal center
- SIGNAL 1 –> in B cell zone
- SIGNAL 2 –> at T-B border
- go to follicle to form Germinal Center (GC reaction), where B cells become plasma cells
what are the 2 ways that plasma cells are formed?
- B cells become plasmablasts then become plasma cells
- directly become plasma cells from B cells
what are plasmablasts? (4 things)
- differentiated B cells that have begun to secrete IgM antibodies (early Ab production)
- but still have cell surface BCRs
- can still proliferate
- arise from primary focus
describe antibodies on naive B cells
naive B cells have cell surface IgM and DO NOT secrete antibody
what are plasma cells?(4 things)
- differentiated B cells that rapidly secrete large numbers of diverse Ab with high affinities
- little to no cell surface immunoglobulin
- cannot proliferate
- arise from germinal center reaction
what are the 2 main outcomes of the primary focus?
- plasmablast
- IgM+ memory B cells
what do plasmablasts produce vs what do IgM+ memory B cells produce?
plasmablasts –> early antibody production (mainly IgM)
IgM+ memory B cells –> only IgM production
where does the primary primary focus form? (3 places)
- near subcapsular zone
- in interfollicular regions (surrounding follicles)
- in medullary cords
what happens to B cells in the primary focus?
undergo proliferation and differentiation into plasmablasts
when do primary foci occur?
5 days after primary infection (i.e. 5 days after post-Ag presentation)
what are the 2 fates of plasmablasts?
- die by apoptosis within 5-10 days after secreting IgM
- migrate to bone marrow to become plasma cells to continue Ab production
what is another name for the germinal center?
Secondary Lymphoid follicle
what are the 2 main outcomes of the germinal center/Secondary Lymphoid follicle?
- plasma cell
- memory B cell
what do plasma cells secrete?
large quantities of antibodies
why are memory B cells important?
important for memory response –> maintain capacity to produce higher affinity antibodies for a secondary antigen exposure
describe what happens in the germinal center/Secondary Lymphoid follicle
- B cells receive signals 1 and 2 AGAIN (2nd time)
- B cells undergo differentiation to make more antibodies that are more effective with higher affinity
- size peaks at 7-12 days after antigen stimulation
what allows B cells to receive signals 1 and 2 again? why do they receive them again?
germinal center has many cell types, including T cells so B cells can be further activated
what are the 3 mechanisms that B cells produce more antibodies that are more effective with higher affinity?
- somatic hypermutation
- affinity maturation
- class switching
where do plasmablasts secrete their antibodies?
mainly in lymph node
where do plasma cells secrete their antibodies?
can either stay in medulla of lymph node or travel to bone marrow and reside there
are antibodies from plasmablasts or plasma cells produced first?
plasmablasts
are antibodies from plasma cells IgM?
no –> they switch classes
what are the 3 general functions of antibodies?
- neutralization
- opsonization
- complement activation
