B cells Flashcards

Question

Characteristics of Plasmablasts

Answer 1

• B cells that secrete antibody • Responsible for early antibody production • IgM produced • Most plasmablasts in primary focus die by apoptosis within 5-10 days • Some plasmablasts can migrate to the bone marrow and become plasma cells, where they continue antibody production

Answer 2

• B cells receive signal 1 and 2 again • B cells undergo differentiation and processes to produce antibodies that are more effective and have higher affinity • The processes involved are Somatic hypermutation, Affinity maturation and class switching • Size of germinal centers peaks 7-12 days after antigen stimulation

Answer 3

Primary focus: Plasmablasts The two main outcomes: 1. Plasmablasts: early antibody production (mainly IgM) 2. IgM+ Memory B cells: Production of IgM

Answer 4

Second phase: germinal centers Germinal centers can also be called Secondary Lymphoid follicle The two main outcomes: 1. Plasma cells: Secrete large quantities of antibodies 2. Memory B cell : Importantt for memory response- maintain capacity to produce higher affinity antibodies ] Plasma cells secrete antibodies with higher affinity

Answer 5

• Antibodies are a Y shaped glycoprotein • An antibody is made up of 2 identical heavy chains(H) that are linked by disulfide bonds and 2 identical light (L)chains, making total of 4 chains • The heavy chain is made of 4 domains while the light chain is only made of 2 domains

Answer 6

Protein family to which antibodies and B-cell receptors belong to -Immunoglobulin molecules are made up of Ig-like domains

Answer 7

• The variable region is the domain located at the top of the Y (antibody) • Both the Light and Heavy chains contain 1 variable region Function : Antigen binding VL and VH form a antigen binding site meaning that on an antibody there are a total of 2 antigen binding site The binding of an antigen can result in the neutralization and other functions

Answer 8

• The constant region is at the bottom of the variable chain. • Both the Light and Heavy chains contain have a constant region. Light chain has 1 constant region and the heavy chain has 3 constant regions Function: involved in Complement Activation (C1q leading to the classical pathway) Constant regions(Fc) can bind to Fc receptors on phagocytes and other cell types such as mast cells and eosinophils leading to the secretion of substances such as histamine

Answer 9

• Fab= Fragment antigen binding • Each has antigen binding domain and part of the constant H and L chains • Two fab fragments per antibody

Answer 10

• Fc= Fragment crystillizable • It is the constant region of the heavy chain • One Fc fragment • Receptors that bind antibodies recognize the Fc region

Answer 11

• Both the constant and variable regions are folded in complex three dimensional structures, including Beta strands

Answer 12

CDR= Complementarity determining region (CDR) • Antigen binding site (direct contact with Ag) • Hyper variable loops- 3 loops per variable domain • Not part of the beta strands CDRs have the greatest variability in the antibody sequence CDRs are at the extremities of the antibody

Answer 13

• The binding of an antigen involves Non-covalent bonding between the Ig and the Ag epitope • Examples of non covalent bonds: Hydrogen bonds, Van der Waals, Hydrophobic, Ionic

Answer 14

• Lock and key specificity meaning that an antibody is specific to an antigen • Extremities of the antibody are responsible for the antigen binding • There is a variation of the size of the antigen which results in a variation of the antibody itself, the CDR varies in length • Location of the epitopes can be anywhere on the antigen

Answer 15

Main difference between those classes is the different number of Ig-like domain resulting in a difference in the length of the constant region of the heavy chain. They are differentiated by their amino acid sequence of their heavy chain (constant region) • Heavy chain is what differentiates the different antibodies-> Fc fragment of each Ig is different

Answer 16

1. IgM • Pentameric meaning 5 IgM units to form a pentagon to then perform their function • 5 antibodies linked together via disulphide bonds • Heavy chain: One variable region and 4 constant regions • Mature naive B cells express transmembrane IgM prior to activation • IgM is part of the first wave of secreted antibodies • Most effective initiator of the complement cascade due to its high affinity with C1q 2. IgD • Heavy chain: one variable region and 3 constant regions • IgD is part of first wave of secreted antibodies Most effective 3. IgG • Heavy chain: one variable region and 3 constant regions • The most abundant in plasma (blood) • Constituted of 4 subclasses in humans- IgG 1, 2, 3, 4 (differ in structure and function) • Produced following differentiation in the Germinal Center, only in this process since germinal center as the Class switching mechanism making the different subclasses 4. IgE • heavy chain: one variable region and 4 constant regions • Produced in response to Helminth infections • They play a role in the function of TH2 5. IgA • Heavy chain: one variable region and 3 constant regions • Monomer in plasma • Dimer in mucous secretions through the J chain • Important for mucosal immunity (Mainly in mucosal surfaces and found as a dimer)- Example: lining of stomach, nose • Two subclasses: IgA1 and IgA2

Answer 17

• this primary diversity happens due to somatic recombination • Primary diversity includes combinatorial diversity and junctional diversity events

Answer 18

• B cells use groups of segments of genes to create different possible antibodies using somatic recombination • Somatic recombination Process by which segments are rearranged • Tightly regulated machinery controls the recombination processes and many of the proteins are involved in DNA repair functions

Answer 19

• Light chain: Variable (V), Joining (J) and Constant (C) region gene • Heavy chain: Variable (V), Diversity (D), Joining (J) and Constant (C) region gene segments

Answer 20

• CDR1 and 2 are encoded in the V segments of light and heavy chains • CDR3, the most variable CDR, is encoded in the joining of V-J segments of light chain and V,D, J gene segments of heavy chain

Answer 21

• Diversity that is the result of different combinations of V, D, and J regions

Answer 22

• Recombination signal sequences (RSSs) flank each antibody gene segment • Recombinase enzymes recognize the RSS • Each gene has a conserved nonamer (9 bp) and heptamer (7 bp) sequence • Either a 12- or a 23-bp spacer sequence lies between the nonamer and heptamer • The spacing and arrangement dictate that a 12-bp RSS must pair with a 23-bp RSS for recombination to occur- this is known as the 12/23 Rule

Answer 23

Recombination signal sequences (RSSs) direct the pairing of different segments together • Light chain: V-J • Heavy chain: D-J and then V-DJ • RSS f Vh has a 23bp spacer • Segments in between the selected ones will be excised Recombination signal sequences (RSSs) allow loping of DNA and binding specific proteins • RSS regions are brought together, creating a loop in the DNA. Th loop part contains the segments that were not selected • RAG= Recombination activating gene • AG-1 and RAG-2 are necessary for recombination, they are responsible for recognizing and cutting DNA at the immunoglobulin-encoding region and the RSS • Covalently closed DNA hairpin ends • Loop is excised (we also call the loop the signal joint), it is no longer on the chromosome and it gets deleted • Coding region of selected V and J regions remain, it is called the Coding Joint

Answer 24

• The light chain is on two different loci, on different chromosomes, with different constant regions (only one will be expressed in the light chain of the antibody) • Two types of chains : Kappa chain and lambda chain (Contributes to diversity) • Each locus includes many different V and J regions • Only one chain will be expressed and will silence the other

Answer 25

• Situated on one locus • Includes many different V, D and J regions • Many different constant regions represent the different isotypes (corresponds to IgM, IgD, IgG, etc)

Answer 26

junctional diversity refers to the addition or the removal of nucleotides during recombination at the junctions

Answer 27

The signal joint is lighted together and discarded leaving behind covalently closed DNA hairpin ends Repair proteins will bind the hairpin **An endonucleoase, Artemis**, opens the DNA hairpins Hairpin cleavage • The hairpin can be opened in three different ways by Artemis • Addition of palindromic (P) nucleotides at overhangs -There is a template which allows DNA repair enzymes to fill in the complementary strand - Complementary strands of DNA read the same in both directions (5’ end or 3’ end) - 5’ to 3’ direction reads TCGA - Reading backwards on the bottom strand (no coding strand) also yields TCGA - Happens mainly in light chain **Exonuclease activity** may remove nucleotides on each side of the coding joint (they remove unpaired nucleotides) **Terminal Deoxynucleotidyl Transferase (TdT) **can add up to 20 N-nucleotides (non-template-encoded) to the cleaved strands primarily in heavy chain- The main reason behind the CDRs variety in length. (Primarily in heavy chain) **Repair enzymes** then trim off non matching nucleotides, fill in remaining single-stranded gaps and ligate the new DNA

Answer 28

• Composed of an alpha chain and Beta chain • Each chain has a Variable and a Constant region • Alpha chain locus has multiple V and J segments • Beta chain locus has multiple V, D and J segments • Somatic recombination takes place in the thymus and is irreversible

Answer 29

• VJ in alpha chain • VDJ in Beta chain (D-J recombination occurs first and then V-DJ) • Combination of alpha and beta chain

Answer 30

• V, D, and J segments are flanked by RSS • RAG-1/2 recognize theses sequences • Artemis cuts DNA hairpins • TdT adds non-coded nucleotides in the joining regions

Answer 31

• Ig heavy chain= D segment is surrounded by two RSS, both with 12-bp spacing • TCR beta chain= D segments have a 5’ 12 bp RSS and a 3’ 23 bp RSS

Answer 32

ALSO- TCR has CDRs too • 3 CDRs per chain • These are the sites with the most diversity • CDR3 is a particularly important source of diversity • CDR1 and 2 are encoded within the V segments of the alpha and beta chains • CDR3 is encoded in the D and J segments of the beta chain and between the V and J segments of the alpha chain

Answer 33

Allelic exclusion ensures that each B cell synthesizes only one allele for a heavy and one allele for a light chain • Once a B cell receptor is expressed on the surface of the developing cell, it send a signal to silence the part of the gene that codes for the other chromosome • Other chromosome becomes methylated and inaccessible to transcription machinery • Genomic silencing of the other chromosome ensures each B cell will only express the same copy of BCRs that have the same specificity

Answer 34

It is due to alternative splicing • B cells produce a long primary mRNA transcript that is differently spliced to yield either of two distinct mRNA molecules • Secreted and transmembrane mRNA are the result of RNA splicing too

Answer 35

It all happens in the Secondary lymphoid follicle aka Germinal Centre • B cells undergo processes to produce antibodies that are more effective: Somatic hypermutation, Affinity maturation and Class switching

Answer 36

Secondary diversification is a process of differentiation that occurs in the Germinal Center once B cells receive Signal 1 and 2 again

Answer 37

1. Somatic hypermutation: a mutation that allows a B cell to have a higher affinity for its antigen while its specificity remains the same 2. Class switching: A process that replaces one heavy chain constant region with one of a different isotype 3. Affinity maturation: Process of selection where mutated B cells that have a high affinity for the antigen survive Note that Somatic hypermuation and class switching ct on already rearranges Ig genes- V(D)J recombination has already occurred in the variable region

Answer 38

- it operates on activated B cells in peripheral lymphoid organs -> in the germina center of the lymph nodes - High rate of point mutations in the V gene sequences that improve Ag binding - This process also occurs during secondary or tertiary responses resulting in the obtention of higher affinity antibodies

Answer 39

• Cytokines secretes by Tfh in germinal center will inform class switching • Example: Type 2 response cytokine IL-4 induces IgE • Only occurs after B cell activation • Irreversible • Class switch recombination guided by switch regions located upstream of each C gene

Answer 40

• Dark zone and light zone • Follicular helper T cells found in the light zone • Follicular dendritic cells (FCDs) retain Ag in the light zone

Answer 41

1. B cells in the dark zone undergo somatic hypermutation 2. Resulting B cells from the dark zone, migrate to the light zone where they compete to bind antigen trapped on follicular dendritic cells. 3. Higher affinity B cells will bind Ag, which is the signal 1. They will internalize ag:BCR and process it to present on MHC II- affinity maturation(Process which selects the mutated B cells with the higher affinity to the Ag) 4. Lower affinity B cell fail to bind Ag meaning that the do not receive signal 1 and side by apoptosis 5. B cells that process Ag and present it on MHC II can interact with Tfh (linked recognition) 6. Signal is received via CD40 7. B cells receive cytokines from Tfh. These cytokines the type of antibodies that needs to be produced -> Class switching 8. B cells can re-enter the dark zone and undergo additional somatic hypermutation

Answer 42

• Plasma cells: They stop expressing high levels of BCR Secrete Ig of the same specificity as the BCR of the progenitor B cell Should bind Ag with higher affinity Secreted Ig can be IgG, IgA, IgE • Memory B cells Express high levels of BCR BCR has the same specificity as progenitor B cell BCR should have higher affinity

Answer 43

AID (activation-induced cytidine deaminase) • Protein responsible for somatic mutations-> deaminate cytidine residues in ssDNA • Cytidine to Uridine which is then removed • Mismatch repair pathway + error-prone polymerase activity-> incorporate any nucleotide into the nick • Produces individual point mutations in Ig heavy- and light-chain variable regions • Some of theses changes->non-productive

Answer 44

Ag-induced selection of B cells with higher affinity • Somatic hypermuataion mainly occurs in the CDR loops of the V regions • B cells that can bind, process, and present more Ag to cells for cytokine assistance survive-> Affinity maturation • Increased antibody affinity with increased exposure

Answer 45

• easier to detect/monitor for B cells than T cells - Antibodies can be measured in a serum - Memory T cells reside in tissues • Mediated by a small and steady number of memory cells -Some of the proliferating at a given time

Answer 46

Ability of the immune system to respond more rapidly and more effectively on a second encounter with an antigen - Depends on adaptive immune mechanisms: -Ag-specific -Memory responses occur after primary response (secondary, tertiary, etc) -Long-lived • Memory leads to resistance to a particular infectious disease upon re-exposure only after having had that disease once before, or after being vaccinated

Answer 47

- More Abs, more cells - Different Abs (higher affinity), different lymphocyte features

Answer 48

• When B cells must receives cytokine signal, transcription is activated upstream of the constant region • DNA is now accessible to AID (Activation-induced cytidine delaminate on ssDNA) • Nicks are made on both strands of DNA • Irreversible • Double stranded breaks in DNA upstream of constant region will be recombined • DS break repair machinery repairs break by cutting out the intervening DNA • Selected region now adjacent to VDJ region

Answer 49

• Class switch recombination occurs within the germinal center after antigen contact (Signal 2 a second time) • Signals for class recombination - B cells must receive costimulatory signals from CD40 to engage in CSR - The cytokine signal receives determines which isotype is selected/produced

Answer 50

Recombination occurs between switch regions - One after the VDJ region - One upstream of the constant region to be recombined

Answer 51

Primary response: - Most IgM-producing cells (some IgD) come from the primary focus - Some B cells will go to the germinal cneter where thry will undergo somatic hypermutation and class switching late in the response Secondary response: -Some memory B cells make IgM -Most memory B cells express IgG (some IgE and IgA) amd will undergo further somatic hypermutation

Answer 52

Repeated immunization lead to: - Increase in affinity of Ab bc of somatic hypermutation and affinity maturation which can lead to a second entry in the germinal center -Memory B cells also express class-switched surface immunoglobulin isotype -Memory B cells express higher levels of MHC class II, CD40 and receptors for survival and proliferation than naïve B cells -Memory B cells circulate through the blood and take up residence in the spleen and lymph nodes

Answer 53

-It helps memory B cells to acquire and present antigens more efficiently to Tfh than naïve B cells -Increases antibody production

Answer 54

Passive: The immune system is not activated but can be confer by the transfer of immune products, **Protection is not permanent** Active: This is when the immune system is ACTIVATED, **it elicits B cell and T cell response** ex: vaccines

Answer 55

It prevents activation of naïve B cells

Answer 56

It is the fact that, once the effective response is acquired, memory cells arre enlisted rather than activating naïve cells that target new, unique epitopes (This can happen with both passive and active immunization)

Answer 57

Adjuvants are used in vaccines to enhance the immune response to a vaccine It is great in situations where the Ag is a weak stimulator as it can promote inflammation recruiting more immune cells= vaccine more effective. It can also slow down Ag release can promote longer interactions = enhancing in effectiveness

Answer 58

There are vaccines containing a virus with multiple mutations preventing it from causing a reaction in immunocompetent humans * Built-in adjuvants, the recognition of PAMPs from virus can trigger immune response

Answer 59

We make them by isolating the targeted virus and put it in a animal culture resulting in the acquirement of mutation until it grows well in the animal culture

Answer 60

It is when the majority of the population is immune to an infectious agent, significantly reducing the pathogen reservoir due to low chance of a susceptible individual contacting an infected individual

Answer 61

*Antibodies mediate the clearance and destruction of pathogen in a variety of ways * Each Ab isotype has distinct properties and traits that enable it to do so

Answer 62

* Family of cell surface receptors that bind to the Fc portion of Igs: *Expressed on **macrophages, granulocytes, DCs, mast cells, B cells, epithelial cells, NK cells,etc.** *Bind Igs in a **class specific manner** not one fits all

Answer 63

When FC receptors bind their antibodies that are bound to an antigen

Answer 64

* Degranulation * Opsonization *Transportation and maintenance of serum levels *ADCC (Antibody-Dependent Cell-mediated Cytotoxicity) Note: Using FcRs allows non-specific immune cells to take advantage of antigen-specific antigens

Answer 65

IgG- Fc gamma R, R binds Fc region of IgG IgA- Fc alpha R IgE- Fc epsilon R

Answer 66

*Neutralization: Binding the Ag and neutralizing its toxic effect *Opsonization: Facilitatimg phagocytosis via FcRs *Complement activation: Through Fc portion, upon binding to Ag *ADCC: Crosslonking antibodies have a cytotoxic functions. involves NK cells and Granulocytes *Transport: Targeting antibodies for transport across different barriers (eg mucosal barriers, placenta)

Answer 67

*All IgG variant bind to Fc receptors, enhancing phagocytosis by macrophages leading to opsonization *Aggregation of binding can allow crosslinking of FcRs, triggering intracellular signalling *Some IgG subclasses are effective at complemetn fixation and some are good mediating ADCC by NK cells

Answer 68

*NK cells have Fc gamma Rs * They recognize the Fc region of IgG antibodies *If these FcRs bind to antibodies on a cell- crosslinking triggers siganlling- NK cell releases toxic granules - target cell dies by apoptosis *Same toxic granules are released in both cases (ADCC and Innate) but activating signal sent to the NK cell is not the same

Answer 69

*Best known for role in allergy and asthma *Also play a role in protection against parasitic helminths and protozoa *Made in very small quantities, but induce potent effects *Degranulation of eosinophils/basophils *Release of molecules such as histamine to damage large pathogens

Answer 70

IgG and IgA

Answer 71

Antibodies exert their function by binding to their target (Toxins, Viruses, Bacteria) to prevent them from binding their target and exert their effects

Answer 72

They are involved in complement activation as C1q (molecule involved in the classical activation pathway) can bind them once they are on a pathogen surface

Answer 73

1. IgG and IgM are involved 2. IgM is the most effective as it has a total of 10 total Ag binding sites which makes it efficient at forming dense Ab-pathogen complexes that are efficiently engulfed by macrophages

Answer 74

FcRs allow the targeting of different Ig classes to different parts of the body *IgA found in mucosal tissues *IgE found near epithelial surfaces *IgG is widely distributed, including to developing fetus *IgM found in blood FcRs are also involved in the transport of Igs across barriers

Answer 75

1. IgD is a minor component of the blood (0.2% of circulating antibodies) but is present at higher levels in secretions of the upper respiratory tract 2. Effector functions * Bind basophils and mast cell leading them to release antimicrobial peptides (AMPs), cytokines and chemokines

Answer 76

B cell developement begins and mainly occurs in the bone marrow and is cmpleted in periphery (including spleen)

Answer 77

Only negative selection required *Negative selection = deletion *Goal: Delection of autoreactive B cells *How? : Proper gene rearrangement of H and L genes to give rise to an Ig that shows self-tolerance

Answer 78

1. Clonal deletion of strongly autoreactive cells, through apoptosis 2. Receptor editing- reactivation of recombination machinery 3. Anergy- induction of nonresponsiveness to further stimuli (even self-antigen stimuli)

Answer 79

Self-Ag are soluble proteins in circulation or presented on stromal cells and other cells

Answer 80

* B cells exported from the bone marrow are still functionally immature so they progress through the spleen for further maturation *Mature, primary cells migrate to the lymphoid follicles -Express high levels of IgM/IgD on their surfaces as well as other receptors -Negative selection here too (peripheral tolerance) -Recirculate between blood and lymphoid organs -Half-life of approximately 4.5 months in periphery

Answer 81

*Receptor editing of potentially autoreactive receptors occurs in light chains * Recombination machinery can be turned back on, this method is a last effort to salvage the rearrangment *Combinatorial diversity as well as junctional diversity can be involved in the self-reactive BCRs

Answer 82

*Central Tolerance : Induction of immune tolerance in primary lymphoid organs (bone marrow, thymus). Involves Clonal deletion, Receptor editing, Clonal anergy *Peripheral Tolerance: induction of tolerance outside primary lymphoid organs. Involves Anergy, Deletion, Immune regulation (Tregs)

Answer 83

Autoimmunity is a defect in tolerance. *Organ-specific autoimmunity: Predominant injury in an organ or tissue *Systemic autoimmunity: injury of many different tissues

Answer 84

*Cell-mediated autoimmunity: Mostly T cell-mediated, sensitive to thymectomy *Antibody-mediated autoimmunity: Mostly Ab-mediated

B cells Flashcards

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