Lecture 21 Flashcards
What are the two different categories of T-independent antigens?
- antigen that has a large number of repeated epitopes (cross links)
- antigen that has fewer epitopes (cross-links a few BCRs, but they have TLR ligands that engage TLR on the B cell)
What are the two types of T-dependent antigens?
- antigen that has fewer epitopes, but has a protein component
- B cell presents a peptide derived from the protein component to get T cell activation
What are the steps in a T-independent antibody response?
- TI-2 antigens have repeated identical antigenic epitopes that cross link BCRs
- TI-1 antigens don’t have repetitive units, so they activate B cells with co-receptor, BCR, and TLR - complement fragments attached to bacteria are recognized by the B cell co-receptor (CR2/CD21)
- B cell activation (add more)
What are the characteristics of Naive FO B cells?
– express IgMloand IgDhi(same antigen specificity),
– express CD21 co-receptor,
– survive for a few months unless activated by antigen,
– are circulating B cells, moving from one lymphoid organ to the next,
– are dependent of BAFF signals for survival,
– make most of the helper T cell–dependent antibody responses to protein
antigens,
– give rise to most of the high-affinity antibodies and memory B cells that
protect people from repeat infections by the same microbes.
What are the steps in a T-dependent B cell antibody response?
- FO B cell recognizes a protein antigen
- activated by Helper T cell
- B cell differentiates into isotype switched, high-affinity antibody producing, memory B cells and plasma cells
What are SCS macrophages and what do they do?
- subcapsular sinus macrophages
- capture larger antigens in the subcapsular sinus and transport them to the B cell follicle
How do SCS macrophages move antigen from the SCS into the follicle?
- translocation: moved along outside the mp
- transcytosis: antigen is endocytosed, moved through the cell and re-displayed outside the cell near the B cell
What happens to subcapsular antigens that are not captured by SCS macrophages?
resident dendritic cells capture them; this kind of DC degrade internalized antigens more slowly, so they can regurgitate intact antigen
How are antigens in immune complexes with antibodies presented to B cells?
They are captured by MZ B cells and follicular DCs via complement receptors, then presented to FO B cells
What initiates B cell responses? What additional signals are needed to activate B cells?
- antigen recognition
- co-receptors (CR2/CD21) and Th cells
BWhat is the DC’s signal 1, 2 and 3 that it gives to the T cell?
- TCR signalling
- Co-stimulatory interaction
- cytokines
Where do B cells get antigen activated? Where do CD4 T cells get antigen activated?
B: follicle
T: paracortex
How do B cells and T cells form cognate pairs?
DC (event coordinator): secretes cytokines that lead B and T cells into the lymphoid tissue
- B cells express CXCR5 and T cells express CCR7
- they are attracted to each other’s cytokines
- B cell has to
present the EXACT peptide/MHC
combination that the dendritic cell used to
activate the CD4 T cell
What are the steps of Th cells activating B cells?
- CD4 T scans the MHC-peptide complex
- CDR1/2 and CDR3 fit
- CD40/40L
- cytokines
What are the two waves of B cell responses to protein antigens?
- extra-follicular focus of T-dependent B cell activation
- generate low-affinity antibodies that can circulate and limit the spread of an infection. - CD4 T cells differentiate into Tfh cells and migrate into the follicle
- then go with B cells to proliferate in the germinal centre
