Flashcards in Ch4: B Cells Deck (240):
proteins which structurally resemble antibodies.
proteins made by lymphocytes and plasma cells, which specifically react with molecules termed antigens (Ags).
All antibodies are what type of protein?
Where did the name antigen come from?
any compound that elicits an immune response
What is an immunogen
Antigen that causes immune response
What is immunogenicity dependent on? (3)
foreignness, size, and complexity
What two types of molecules induce strong immune response?
Proteins and polysaccharides
Recognition of antigen directed at what?
antigenic determinant or epitope
What is an epitope?
The chemical structures of a molecule that is directly recognized
Antibodies are composed of what?
heavy and light chains that both contain variable and constant regions
Antibodies are made up of how many chains?
2 identical heavy chains
2 identical light chains
Antigens are bound to what part of antibody?
What holds the antigen in place?
fragment antigenbinding (Fab) region
What part of the antibody tells the host how to handle the antigen:antibody complex?
The fragment crystallizable (Fc) region
The interactions of antibodies and antigen are mediated by what types of forces?
1) electrostatic forces,
2) hydrogen bonds,
3) Van der Waals forces,
4) hydrophobic forces
Since the interactions are less stable between antigens and antibodies (non-covalent) what does this allow the antibody to do?
bind multiple sites on the pathogen
How many sites can each antibody bind?
Linear epiotopes can be predicted by what?
Amino acid sequence
Discontinuous epitomes can be predicted by what?
Amino acid sequence AND 3-D folding knowledge
IgA has how many subtypes?
What is the heavy chain of IgA?
Alpha (1 or 2)
Secreted form of IgA?
Monomer or Dimer
Function of IgA?
H chain of IgD?
Secreted form of IgD?
Function of IgD?
Native B cell antigen receptor
H chain of IgE?
Secreted form of IgE?
Function of IgE? (2)
1. Mast cell activation
2. Defense against helminthic parasites
H chain type of IgG?
Secreted form of IgG?
Function of IgG? 5
2. Complement activation
3. Antibody-dependent cell-mediated cytotoxicity
4. Neonatal immunity
5. Feedback inhibition of B cells
IgM H chain type?
Secreted form of IgM?
Function of IgM? (2)
1. Native B cell antigen receptor
2. Complement activation
B cells are produced where?
By what cell?
Hematopoietic stem cell
The germline DNA contains multiple copies of what types of genes that will be rearranged? 3
variable (V), diversity (D), and joining (J) genes
Why does gene rearrangement occur for B cells?
provide significant variability to the antigen-binding portion of immunoglobulins
The heavy chain uses what germline genes?
The light chain uses what germline genes?
Successful rearrangement of the VDJ (heavy chain) and VJ (light chain) allows these genes to do what?
associate with constant genes, and the rearrangement is complete
Diversity in T and B cells means what?
The number of antigen specificities present within a single individual at any given time
B cell germline rearrangements occur when?
Do we have immunoglobulins for antigens we have not encountered?
How many Variable gene segments exist for light chains?
How many variable gene segments for heavy chain?
How many diversity segments in light chains?
How many diversity segments in heavy chains?
How many joining segments in light chains?
How many joining segments in heavy chains?
How many constant segments in light chains?
Which gene rearrangements occur first, light or heavy?
Heavy (D to J) then (V to DJ)
Gene rearrangements for light chain involve what segments?
V to J fusion
Each V, D, or J segments is flanked by what?
Recombination Signal Sequences (RSS)
Two types of Recombination Signal Sequences
1. Nonamer and Heptamer separated by a 12 base pair segments
2. Nonamer and heptamer separated by a 23 bp segments
The 12/23 rule ensures what?
Gene segments are joined in the correct orientation
During rearrangement of gene segments, what two genes bind to the 12 bp and 23 bp spacers?
Function of this?
RAG--1 and RAG-2
Bring two heptamer sequences in close proximity through formation of a hairpin that contains the DNA segments to be removed
DNA is excised where in germline rearrangment?
End of both heptamer sequences
What joins chromosomal DNA together following recombination?
Terminal deoxynucleotidal transferase (TdT)
Describe the steps of Germline rearrangement 8
1. Generation of junctional diversity
2. RAG complex cleaves heptamer RSS's from the D and J gene segments to yield hairpins
3. RAG complex opens hairpins by nicking one strand of DNA which generates palindromic P-nucleotides
4. N-nucleotide additions are performed by TdT
5. TdT adds N nucleotides
6. Pairing of strands occurs
7. Unpaired nucleotides are removed by exonuclease
8. Gaps are filled by DNA synthesis and ligation to form coding joint
What are P nucleotides?
pallindromic nucleotides that are identical when read from either end, but they are not complementary to each other and cannot join together.
What are N nucleotides?
Nontemplated/not encoded in germline DNA
How does TdT contribute to diversity of antibodies?
Formation of coding joint involves TdT adding N-nucleotides
How might two B cells with identical V, D, and J genes differ?
Different N-nucleotides added
After B cells rearrange the heavy chain, they express what?
For what reason?
Express a Pre-B cell receptor
To test the ability to express the heavy chain at the cell surface
What is needed to make the Pre-B cell receptor?
1. Heavy Chain
2. Surrogate light chain
3. Two disulfide linked signaling molecules (Igalpha and IgBeta)
B cells that possess an appropriate heavy chain in the Pre-B cell receptor undergo what?
cell division to produce around 100 clones expressing the same heavy chain
Do clones of B cells have the same light chain?
No. each clone begins making light chains independently
How many chances does a B cell have to create a proper light chain?
both κ light chain and λ light chain gene rearrangements are possible
If a successful light chain rearrangement does not occur, what happens to the B cell?
Dies by apoptosis
If a successful light chain rearrangement does occur, what happens?
the B cell receptor is expressed at the cell surface along with two disulfide-linked signaling molecules (Igα and Igβ)
As an early pro-B cell, what can the B cell do for rearrangements?
D-J rearrangements on both chromosomes
As a late pro-B cell, what can the B cell do for rearrangements?
And if this fails?
And if this fails?
1. V-DJ rearrangement on first chromosome
2. V-DJ rearrangement on second chromosome
If successful rearrangement occurs of V-DJ, what can the B cell do next for light chain rearrangement?
If this fails?
If this fails?
If this fails?
If this fails?
1. Rearrange Kappa gene on first chromosome
2. Rearrange Kappa gene on second chromosome
3. Rearrange Lambda gene on first chromosome
4. Rearrange lambda gene on second chromosome
If successful kappa light chain rearrangement occurs in Pre-B cell, what is the result?
Cell expresses IgM of mu/Kappa
If successful lambda light chain rearrangement occurs in Pre-B cell, what is the result?
Cell expresses IgM of mu/Lambda
What are the two checkpoints in determining whether a B cell will make functional IgM?
1. First: Can it make a pre-B cell receptor
2. Second: Can it make a mature B cell receptor
What is allelic exclusion?
process of encoding immunoglobulin genes from a single chromosome per cell, instead of both
What is affinity?
Strength of binding at a single site
What is avidity?
Strength associated with binding multiple
sites, regardless of affinity
Having multiple B-cell receptors that recognize the same antigen increases what?
increases the avidity and potential for activation of the B cell because multiple signals tell B cell to become activated
After the V, D, and J genes rearrange, the B cell can proceed to rearragnements with what genes?
How many constant genes exist on heavy chain of B cells?
What do they determine
define the antibody isotype that is being produced
by the cell.
The default constant gene for all B cells is what? (2)
Cμ (IgM) and Cδ (IgD)
B cell express what antibodies at cell surface during development?
IgM and IgD
What two types of IgM do B cells express?
surface IgM and secreted IgM
Downstream constant genes (not the default ones) exist for what reason?
Future isotype switching
How many different V gene segments exist for B cell heavy chains?
How many different V gene segments exist for B cells' kappa chains?
How many different D gene segments exist for B heavy chains?
How many different D gene segments exist for B kappa chains?
How many different J gene segments exist for B kappa chains?
How many different J gene segments exist for B heavy chains?
Which has the better possible repertoire of antigen diversity, TcR's or Immunoglobulin?
TcR's 10^16 vs 10^11
The primary transcript for antibody is processed by what 3 things?
cleavage, polyadenylation, and splicing
Processing of primary antibody transcript creates what? 2
1. μ transcript that does not encode the δ,
2. δ transcript that does not encode μ.
The mu transcript and delta transcript creation allow for expression of what?
expression of both IgM and IgD surface immunoglobulin by naïve, mature B cells
Do some B cells secrete IgD?
yes but not commonly
What mediates association of lymphoid progenitor cells with stromal cells of bone marrow?
Specifically what? (2)
cellular adhesion molecules
1. Lymphoid VLA-4 (integrin)
2. Stromal cell VCAM-1
B cells express what protein that binds to membrane-bound stem cell factor?
Production of IL-7 by bone marrow stromal cells has what effect?
stimulates B cell growth and proliferation
Function of Kit, IL-7 and CD25 on pro B cels?
Growth factor receptors
Function of RAG-1 and RAG-2 in development of B cells
Lymphoid specific recombinase
Funtion of TdT in pro and pre B cells?
What are Lambda5 and VpreB's functions in B cell development?
Surrogate light chain components
Ig-alpha and Ig-Beta, BtK, CD19, CD45R are involved in what during B cell development?
CD43, CD24, and BP-1 have what function in B cells?
E2A and EBF along with Pax-5 have what function in B cell development?
What happens if immature B cell has no reaction with a self antigen in bone marrow?
Moves to the blood and expresses IgD and IgM
What happens if immature B cell has a reaction with a self antigen in bone marrow?
Retained in bone marrow
What happens to B cells that are self-reactive in bone marrow?
undergo receptor editing
how does receptor editing occur?
rearrangement of the light chain locus, to express a new Ig V region
The current, self-reactive rearrangement is deleted, and rearrangement of the light chain continues until the B cell is either non-self reactive or no V regions are available
Low avidity self antigens induce what?
Anergic B cells are removed from circulation how quickly?
Non-self-reactive B cell lasts how long in blood?
40-day half-life in blood.
What is central tolerance?
elimination of self-reactive B cells before they leave the bone marrow
What is peripheral tolerance?
elimination of self-reactive B cells after they leave the bone marrow
What is also responsible for differential expression of the transmembrane and secreted forms of IgM?
Alternative RNA processing
Each transcript has how many polyadenylation sites?
1. one after the secretion-coding sequence (pAμs)
2. one after the membrane-coding sequence (pAμm).
Secreted IgM, which is linked by what molecule?
Forms what structure?
How many antigens can IgM pentamer bind?
What is IgM's avidity?
What is IgM's affinity?
B cells become activated when?
When their receptor are cross-linked by antigens
Cross linking induces signaling with B cells which is mediated by what?
immunoreceptor tyrosine-based activation motif (ITAM) signaling components of the B cell receptor (Igα and Igβ)
What exactly happens to the ITAM's?
phosphorylated by the receptor-associated
tyrosine kinases Blk, Fyn, and/or Lyn
After phosphorylation of the ITAM's what happens?
tyrosine kinase Syk then binds the phosphorylated Igβ and initiates intracellular signaling pathways that lead to changes in gene expression in the nucleus
Since a pathogen might not express multiple copies of an epitope, what do B cells have in addition to cross-linkign to activate themselves?
Co-receptor which consists of proteins (CR2, CD19, and CD81)
Explain the steps of the B-cell co-receptor in action?
1. CR1 on the B cell surface binds to pathogens coated with C3b
2. Factor I cleaves C3b into iC3b
3. iC3b is cleaved into C3d
4. CR2 recognizes either iC3b or C3d and stimulates signaling through CD19
Complement decorating a pathogen provides what to the B cell?
provides a signal to a B cell recognizing a specific antigen
What does complement tell the B cell?
Complement tells the B cell that the antigen is foreign and bridges innate and adaptive immunity
CR2 also is known by what name?
CR2 or CD21 recognize what?
IC3b and C3d
What is the signaling molecule of CR2?
What attracts B cells to the high endothelial venules?
Chemokine CCL21 interacts with receptor CCR7 on B cells to attract B cells
What attracts B cells to the lymph node?
CCL21 and CCL19
What attracts B cells into primary follicle?
What two molecules drive the mature of immature B cells in the primary follicle?
1. B-cell activating factor in the TNF family (BAFF)
2. Lymphotoxin (LT)
What family of molecules do BAFF and LT belong to?
BAFF is produced by what cells?
follicular dendritic cells
BAFF binds to its receptor on B cells to promote what?
survival and maturation of the B cell
LT is produced by what cells?
LT binds to its receptor on follicular dendritic cells to do what?
preserve integrity of the FDC network
Mature B cells recirculate between what?
Lymph, blood, and secondary lymphoid tissues
4 main responses of B cell to bound antigen?
And result of each
1. Entry into cell cycle (Clonal expansion)
2. Increased expression of cytokine receptors (Respond to cytokines made by helper T cells)
3. Migration out of lymphoid follicles (Interact with helper T cells)
4. Secrete low levels of IgM (Early phase of humoral immune response)
4 general steps of B cell antigen presentation?
1. B cell recognizes native protein antigen
2. B cell undegoes receptor mediated endocytosis of antigen
3. Antigen processing and presentation
4. Helper T cell recognizes antigen
What leads to antigen uptake by B cell?
When an antigen binds its antigen-specific B cell receptor and activates it
How does a B cell present its processed peptide epitopes to a helper T cell?
What recognizes the B cell's peptide:MHC complex?
Result upon this recognition?
The antigen-specific T cell receptor
expresses CD40 ligand on surface
CD40L ligand of the helper T cell binds to what?
What does this provide?
CD40 on B cells
provides a second signal for B cell
activation, leading to division and differentiation of B cells
All the while CD40 is doing its thing, what else is the T cell doing?
produce cytokines to assist in this activation, with the ultimate goal of inducing antibody production from the B cell.
Where is the primary focus of expansion of antigen activated B cells?
The secondary focus of expansion of antigen-activated B cells is where?
Activated B and T cells migrate out of T Cell zone of lymph node and go to where?
B cells in medullary cords produce what?
Where does this go?
exits via the efferent lymphatic vessel, gets into
the blood, and rapidly travels to the site of infection
Serum IgM antibody is first detected when?
After what happens?
7-10 days after antigen exposure
bind 10 antigens and efficiently fix complement
What happens to some antigen-specific B and T cells in the medullary cords?
migrate back to a primary follicle and enter into
the germinal center reaction
T cells are activated with what?
Where do T and B cells typically have first interaction?
in the T cell zone of the lymph node
B cells can also present antigen in order to match up with what?
activated T cell
B cells that encounter their antigen form what?
And then go where?
Where they under go what? (2)
germinal centers and enter into the germinal center reaction
1. Affinity Maturation (somatic hypermutation)
2. isotype switching
What maintain the antigen during germinal center reactions?
Via what receptor?
Follicular Dendritic Cells
After B cells produce antibodies, they bind what on FDC's?
In order to do what?
Fc receptors on FDCs
hold noncomplement-bound epitopes in place on the FDC
Somatic hypermutation targets what?
Rearranged gene segments encoding the variable region
What happens in somatic hypermutation?
The enzyme activation-induced cytidine deaminase (AID) deaminates cytosine residues within the single-stranded DNA into uracil, and DNA repair
mechanisms randomly converts the uracil to any one of the 4 bases found in DNA
What cells make AID enzyme?
Proliferating B cells
What is purpose of hypermutation?
increase the affinity and improve specificity of antibody toward the invading pathogen
What is isotype switching?
the process by which a B cell changes the constant region that is expressed in its germline
Isotype switching is done how? 2
1. AID deaminates cytosine to uracil at two sites in the DNA
2. Uracil is removed by uracil-DNA-glycosylase and recombination occurs
Isotype-switched cells now produce what?
Whatever the next remaining downstream constant segment is? (gamma, epsilon, alpha)
What happens to constant regions removed by isotype switching?
Since all B cells start as IgM producers, what happens to IgM expression throughout an immune response?
Reduces because cells switch to other isotypes
What will cause an antigen-selected B cell centrocyte to form into plasma cells?
IL-10 secreted from helper T cell
What will cause an antigen-selected B cell centrocyte to form memory cells?
IL-4 secreted from Helper T cell
What do plasma cells do?
Make antibodies that fight and terminate current infection?
What do memory B cells do?
Prevent future infections from causing disease
Primary response takes how long?
Secondary response takes how long?
What is relative difference between primary and secondary peak response?
Secondary rseponse has larger peak response
What is the balance of IgM and IgG in primary response?
IgM > IgG
What is balance of IgG and IgM in secondary response?
IgG > IgM
What is relative difference between antibody affinity in primary response and secondary response?
Secondary response has higher average antibody affinity than primary
What two types of B cells are in spleen and other lymphoid organs?
Follicular B cells
Marginal Zone B cells
What type of B cels are in mucosal tissues?
Both marginal zone B cells and B-1 cells secrete what mainly?
What do they differentiate into?
Short-lived plasma cells
Two types of T-independent antigens?
TI-1 antigen has what main example?
LPS on gram negative bacteria
Thymus independent antigens can do what?
activate B cells without T cell
help in one of three ways
What are 3 ways a TI-1 antigen will activate a B cell?
1. The TI-1 antigen directly activates the B cells response to that specific antigen
2. TI-1 antigen activates B cells that are specific for other antigens
3. The DNA can act as TI-1 that activates B cells specific for a surface antigen
TI-1 activation of B cells is dependent on what?
complement components and TLR recognition of
TI-2 antigens activate B cells how?
Cross-linking the BCR and its co-receptor due to repetitive expression of identical epitopes on the surface of the pathogen
TI-2 crosslinking is so strong that what is not needed?
Do TI-2 antigens use complement components?
How about TLR recognition?
What cells typically respond to TI-2 antigens?
What antibodies circulate through the bloodstream? (3)
1. Pentamer IgM
2. Monomeric IgA
What are the main isotypes in extracellular fluid?
1. Monomeric IgA
What antibody coats mucosal surfaces?
What antibody associates with mast cells in the CT?
Passive transfer of IgG from mother to fetus occurs across the placenta via what?
What allows dimeric A to be in mother's breastmilk?
What mucosal cell receptor binds dimeric IgA?
Via what portion?
For what purpose?
Polymeric Ig Receptor (pIgR)
Transport IgA across mucosal surface
What part of pIgR is cleaved and released along with IgA?
Two functions of secretory component?
1. Binds to mucins and holds IgA in the mucus layer
2. Protects IgA from degradation by enzymes present at mucosal sites
pIgR has a higher affinity for IgA or IgM?
People with an IgA deficiency will transport what instead across mucosal surfaces?
FcRn in placenta will do what?
Transport IgG from mother to fetus
FcRn is expressed by endothelial cells and extends the half life of IgG to what?
At birth, what antibody do newborns have the most of?
IgG from the mother
What happens as passive IgG from mother begins to wane? 2
1. Newly synthesized IgM dominate
2. Infant produces its own IgG at 6 months of age
Newborns do not have memory cells against pathogens until when? 2
2. infected with pathogen
Interaction with a pathogen is done with what part of the antibody?
Fab (variable region)
Interaction with a host cell is done with what part of antibody?
What role do antibodies whose Fc regions do not have high affinity interaction with Fc receptors have?
protecting the host by neutralizing the pathogen
so that it cannot interact with the host cell
Toxins naturally interact with cell surface receptors before what?
being endocytosed and releasing their toxic
How do IgG antibodies that are bonded with an antigen on a pathogen help complement?
C1q binds to two or more IgG molecules and initiates complement activation
How do IgM antibodies that are bonded to soluble multivalent antigen help complement?
C1q binds to the complex and initiates complement activation
When immune complexes bind C1q and fix c3b they can be recognized by what?
CR1 on erythrocytes
The erythrocyte with CR1 can transport immune complex to where and for what?
To liver or spleen where it is detached and taken up by macropahges
The purpose of erythrocytes with CR1 binding immune complexes is what?
Keeping complexes from depositing in the kidney glomeruli
What do Fc receptors on host cells allow?
induction of an active process associated with removal of a pathogen from an infected host
What is the high affinity receptor for IgG1 and IgG3?
What portion of Fc-gamma-RI does the Fc bind to?
What portion performs the signaling?
What does this signaling utilize?
Extracellular alpha chain
Activation of Fc receptors requires what?
cross-linking of these receptors
Explain osphonophagocytosis with IgG! 5
1. Antibody binds to bacterium
2. Antibody coated bacterium binds to Fc receptors on cell
3. Macrophage membrane surrounds bacterium
4. Macrophages membranes fuse to form phagosome
5. Lysosomes fuse with phagosome to form phagolysosome
What antibody-dependent cell-mediated cytotoxicity
When IgG binds to surface of target cell can then links with Fc receptor on NK cells (FC-gamma-RIII) which causes NK cells to release cytotoxic granules of perforin and granzymes to kill target cell
The high affinity Fc receptor for IgE (FC-epsilon-RI) is expressed where?
What causes an eosinophil to be ready for IgE bound parasites?
IL-5 released from Th-2 cell
6 main functions of IgG
1. Neutralize microbes and toxins
2. Osponization of antigens for phagocytosis
3. Activation of classical complement pathway
4. ADCC mediated by NK cells
5. Neonatal immunity
6. Feedback inhibition of B cell activation
Main function of IgM?
Activation of classical pathway of complement
Function of IgA?
Function of IgE? (2)
1. Defense against helminths
2. Mast cell degranulation
What main Fc receptor acts as an inhibitory response to downregulate antibody responses?
Fc-gamma-RIIB sends a negative signal through what?
B cell to stop making antibody
First pregnancy of Rh- mother with Rh+ fetus results in what? 3
1. Primary immune response of IgM with low affinity IgG
2. Minor destructin of fetal erthryocytes by anti-Rh IgG
3. health newborn
Second and subsequent pregnancy of Rh- mother with Rh+ fetus results in what? 3
1. Secondary immune response of high affinity IgG transcytosed to fetal circulation
2. Massive destruction of fetal erythrocytes by anti Rh igG
3. Anemic babies
First pregnancy of Rh- mother with Rh+ fetus infused with anti-Rh IgG results in what? 3
1. Primary immune response to Rh is inhibited by presence of Rh-specific IgG
2. Fetal erythrocytes are not destroyed
3. Healthy newborns
Fc-gamma-RI has what affinity for Ig?
Specifically what Ig's
What cells have it? (3)
IgG1 and IgG3
Macrophages, Neutrophils, Eosinophils
Fc-gamma-RIIA has what affinity for Ig?
What cells have it? (3)
Macrophages, Neutrophils, Eosinophils
Fc-gamma-RIIB has what affinity for Ig?
What cells have it? (4)
B cells, Dendritic cells, mast cells, neutrophils, macropahges
Feedback inhibition of B cell and attenuation of inflammation
Fc-gamma-RIIIA has what affinity for Ig?
What cells have it?