Immunology: Chapter 10

Question 1

T-cell dependent

Answer 1

Require specific T cell help
Thymus-dependent (TD) –> do not occur in animals that lack T cells
1.) Cross-linking antigen binds to the surface of immunoglobulin
2.) Antigen is internalized + returned to the cell surface as peptide:MHC class II complexes
3.) Helper T cells bind to naïve B cells
4.) B cell is induced to proliferate and differentiate into antibody-producing plasma cells

Question 2

T-cell independent

Answer 2

Bacterial constituents (i.e. polysaccharides) can induce antibody production in absence of helper T cells
Thymus-independent, TI antigens
Second signal required to activate antibody production delivered by either:
Recognition of common microbial constituent, OR
Massive cross-linking of B-cell receptors

Question 3

CD:19:CD:21:CD81 co-receptor complex

Answer 3

CD21: (extracellular domain) receptor for complement fragments C3d ad C3dg
Complement is activated –> complement fragments may be directly deposited on an antigen for which a B cell is specific
Co-receptor complex is linked to BCR
Upon linking, CD19 activated a PI 3-kinase signaling pathway:
Amplifies B cell activation (increases synthesis of co-stimulatory molecules on B cell, making it more receptive to T cell help)
Increases antibody production
Stimulates antigen-uptake by the B cell

Question 4

Linked Recognition

Answer 4

Helper T cells activate B cells that recognize the same antigen
B cells and T cells:
Respond to the same antigen
Do NOT have the same antigen specificity (responsive to different protein)
Helps prevent autoimmune disease

Question 5

Autoimmune disease

Answer 5

Occurs only if a self-reactive B cell and a self-reactive T cell are present at the same time

Question 6

Haptens and Penicillin sensitivity

Answer 6

Small molecules that can’t cross-link BCRs + can’t elicit antibody production on their own
Can become chemically linked to career proteins at multiple sites
Haptens in linked complexes can cross-link receptors and elicit an antibody response
T-cell dependent processes are possible
T cells primed to peptides from carrier protein can bind to B cells that present peptides as class II MHC complexes
Coupling of hapten to a protein –> allergic response
Penicillin-specific B cells internalize cross-linked penicillin-protein complexes.
Peptide fragments are presented to helper T cells, resulting in prolif + diff of penicillin-specific B cells into plasma cells.
Result is a potent allergic reaction to penicillin (hapten)

Question 7

Infants and meningitis

Answer 7

Meningitis immunity is produced through TI response
Infants are sensitive to bacteria with polysaccharide coats bc TI-responses are necessary for antibody production
Adults, NOT infants make effective TI responses to these antigens
Polysaccharides do NOT generate peptides that can be presented to T cells
Vaccination: polysaccharide is chemically linked to tetanus toxoid (foreign protein)
B cells that bind polysaccharide component can be helped by T cells that respond to the tetanus toxoid component of the vaccine

Question 8

T cells activate B cells

Answer 8

Engage B cells through TCRs
Helper T cells synthesize both cell-bound and secreted effector molecules
CD40 ligand (CD40L) on T cells
Binds to CD40 on B cell
IL-4 (soluble cytokine) secreted by TFH and TH2 cells in response to antigen-specific recognition of B cell
Secreted into target B cells
Helper T cell reorients its cytoskeleton and Golgi to the site of contact with B cells
IL-4 molecules are secreted at the site of binding to B cell (sealed by ring of adhesion molecules)
Helper T can also help activate nearby B cells

Question 9

BAFF

Answer 9

Secreted by dendritic cells and macrophages
Soluble TNF-family cytokine
Survival factor for differentiating B cells

Question 10

How do B cells and helper T cells find each other?

Answer 10

Only 1 in 10^8 to 1 in 10^12 encounters will be correct
Circulating naive B cells enter primary lymphoid follicles, attracted by the chemokine CXCL13
Secreted by FDCs and stromal cells
Antigens coated with C3b and C3dg can enter follicle and be trapped there by complement receptors CR1 and CR2 (expressed by FDCs).
Can also be trapped by specialized macrophages adjacent to follicles and sometimes be transferred to FDCs

Question 11

Opsonized antigens

Answer 11

Enter lymph nodes from afferent lymphatics and bind to the complement receptors on the surface of macrophages present in the sub capsular sinus
Antigen is retained on the surface of the macrophages, allowing B cells to encounter them
Also allows antigen to be transported into the follicle to become localized on the surface of FDCs

Question 12

B and T cell meeting

Answer 12

When naive B cel in follicle meets specific antigen on FDC, it expresses chemokines that make it come toward T-cell area
Activation leads to decrease in S1P1 on B-cell surface, which retains the cell in the lymphoid tissue
When Naive T cell encounters cognate peptide antigen on dendritic cells, it begins to proliferate
Some differentiate into effector T cells and leave lymphoid tissue
Others become TFH cells and migrate to boundary of T-cell area with the follicle
ONLY antigen-bound T cells look for partners among antigen-bound B cells
Activated B cells that fail to receive T cell help die within 24 hours

Question 13

Primary Focus

Answer 13

Formed by T cells and B cells
Spleen: located at border of T cell zone and red pulp
Lymph nodes: in the medullary cords where lymph drains out of the node
Appear within 5 days after infection with a new antigen
Some may leave primary focus and migrate to a lymphoid follicle to establish a germinal center for further differentiation
May differentiate into plasmablasts
Do NOT migrate to follicle
Stop dividing after few days and die
Differentiate further into plasma cells
Some remain in lymphoid organs and are short lived
Majority migrate to bone marrow and continue to antibody production there

Question 14

Resting B cells

Answer 14

Still dividing
Make high levels of the BCR and MHC class II molecules
High Surface Ig
Class (isotype switching)

Question 15

Plasmablasts

Answer 15

Still dividing
Properties intermediate between resting B cells and plasma cells
Make high levels of the BCR and MHC class II molecules
High suface Ig
Secrete antibody
Prominent Golgi and abundant cytoplasm
Class (isotype) switching

Question 16

Plasma Cells

Answer 16

Secrete antibody
May live for decades or only for a few days 
No MHC molecules on surface
Low surface Ig
Do not divide
Can't present antigen to helper T cells

Question 17

2nd Phase of Primary B-cell immune response

Answer 17

Absence of infection: primary follicles consist of resting B cells clustered around FDCs
FDCs secrete a chemokine that attracts both naive and activated B cells into follicles
Some B cells and their associated T cells migrate to a primary lymphoid follicle

Question 18

Germinal Centers

Answer 18

Composed mainly of proliferating B cells
10% antigen specific T cells
Germinal B cells continue to multiply and displace resting B cells towards the periphery of the follicle –> mantle zone
Germinal center of rapidly growing B cells is established as a secondary follicle
Dark zone: rapidly proliferating B cells called centroblasts
Light zone: less rapidly proliferating B cells
Grow in size as the immune response proceeds, then shrinks and disappears after infection is cleared
Present ~3-4 weeks after initial antigen exposure

Question 19

Contrast

Answer 19

Early events in primary focus gives early production of antibody with relatively low affinity –> prompt
Germinal center takes longer, but results in high-affinity antibodies

Question 20

B cells in Germinal centers

Answer 20

Somatic hypermutation
Affinity maturation (positive selection for survival of B cells with high affinity for antigen)
Isotype switching
Some B cells differentiate into memory B cells, or
Plasma cells (higher affinity and isotope-switched antibody)
B cells divide every 6-8 hours

Question 21

Somatic Hypermutation

Answer 21

Occurs only in germinal centers
In each generation, 1/2 the B cells acquire single amino acid substitution
Most are deleterious (chain termination, altered folding, or unacceptable changes in the complementarity-determining region)
These cells are eliminated by apoptosis

Question 22

Mutations –> apoptosis

Answer 22

No longer make functional BCR (i.e. antibody)
Cant compete with BCRs that bind antigen more strongly
Strong antigen binding- cross-linked by antigen- results in more T cell help
Reduced antigen binding –> reduced/ no T cell help –> apoptosis
B cells divide every 6-8 hours
Massive dying off of B cells (germinal centers are filled w/ apoptotic B cells that are quickly engulfed by macrophages)

Question 23

Dark and Light zones

Answer 23

Somatic hypermutation occurs in centroblasts that occupy the dark zone
Reduces its rate of proliferation
Increases expression of the new antigen receptor on its surface
Migrates to the light zone and becomes a centrocyte
Selection occurs in the light zone
Reacts strongly to antigen –> reenters the dark zone as centroblasts
Do not react (can’t compete with other centrocytes) –> apoptosis
**Cyclic reentry cycle

Question 24

Cyclic Reentry Model

Answer 24

Over time, numerous amino acids substitutions are made in the complementarity-determining regions
~50-100 B cells seed a germinal center
Only 1 to several leave progeny
Intense selection
If descendants are from one B cell, all antibody molecules after selection are identical

Question 25

Class Switching

Answer 25

Occurs in germinal centers In thymus-dependent antibody responses, requires expression of CD40 ligand by the helper T cell and is directed by cytokines CD4 TH2 cells: produce IL-4 and IL-5 CD4 TH1: produce IFN-gamma Treg: TGF-Beta Individual B cells undergo switching to the same C gene on both chromosomes

Question 26

IL-4

Answer 26

Promotes switching to IgG1 and IgE

Question 27

Il-5

Answer 27

Promotes secretion of IgA by cells that have already undergone class switching to IgA

Question 28

IFN-gamma

Answer 28

Promotes switching to IgG3 and IgG2a

Question 29

TGF-beta

Answer 29

Promotes switching to IgG2b and IgA

Question 30

B cells in germinal centers are inherently prone to die.

Answer 30

Survival requires: Antigen (in a form that cross-links the BCRs) Ligation of CD40L from TFH cells Prolonged contact with TFH cells is required

Question 31

Some B cells eventually exit light zone and start to differentiate into plasma cells that produce large amounts of antibody

Answer 31

Plasma cells migrate to The bone marrow Some live a very long time with aid of survival signals from stromal cells Produce long-lasting, high-affinity antibody Splenic red pulp Not long-lived Those that originated in the germinal centers of the mucosal system stay within the mucosal system and secrete IgA (mostly).

Question 32

Memory B cells

Answer 32

``` Divide rarely, if ever Are long-lived Express surface immunoglobulin Do not secrete antibody at high rate Inherit somatic mutations and class switch changes that occurred in germinal centers ```

Question 33

Thymus-independent (TI) antigens

Answer 33

``` Helper T cells not required TI antigens include: Bacterial polysaccharides Lipopolysaccharides Do not have a protein component and can not be presented on T cells Polymeric proteins Two types: TI-1 and TI-2 ```

Question 34

TI-1 antigens

Answer 34

Possess intrinsic activity that can directly induce B-cell division Can activate either mature or immature B cells Has an epitope that recognizes a specific BCR Has a common microbial constituent that ALL B cells can recognize (using a receptor that's different from the antigen receptor) At high concentrations (never found physiologically) all B cells are engaged and begin to proliferate and differentiate (regardless of specificity) Polyclonal activation Weak binding B-cell mitogens. At low concentrations the antigen-specific B cells are preferentially activated and proliferate without T cell help. There aren't enough molecules to bind to all B cells Exposure to pathogens in vivo Important for defense against several extracellular pathogens. Responses are faster b/c prior priming and clonal expansion of helper T cells is not required. Inefficient inducers of: Isotype switching Affinity maturation (somatic hypermutation) Memory B cells.

Question 35

iNKT cells can help TI antigen responses

Answer 35

``` iNKT cells recognize certain polysaccharides bound to CD1 (MHC class I-like molecules) and are activated by them. Some polysaccharides are TI-1 antigens. Activated iNKT cells can provide help to B cells responsive to TI-1 antigens. ```

Question 36

TI-2 antigens

Answer 36

Molecules such as bacterial polysaccharides that have highly repetitive structures No intrinsic B-cell stimulating activity Can activate ONLY mature B cells Immature B cells are inactivated by repetitive epitopes Most of infant’s B cells are immature; infants can’t make antibodies to TI-2 antigens such as polysaccharide antigens well. Responses to TI-2 antigens are prominent among B-1 cells (present early in development) and marginal zone B cells (Rare at birth and develop in baby’s 1st years). Infants can’t mount TI-2 responses; presume that physiological TI-2 responses come from marginal zone B cells Dendritic cells + macrophages provide co-stimulatory signals for TI-2 antigens. Right amount of cross-linking is required Too little = no response Too much = anergy or apoptosis. TI-2 antigens responses include class switching w/ cytokines such as BAFF from dendritic cells (induces IgM IgG) Important for pathogens in polysaccharide capsules, which resist ingestion by phagocytes and resist standard T-cell dependent responses. TI-2 responses give rapid antibody production, which osponizes bacteria so that they can be recognized + destroyed by phagocytic cells. People who can’t make antibodies to polysaccharide antigens are highly susceptible to infection with encapsulated bacteria.

Question 37

Summary

Answer 37

TD: Antibody response occurs in infants, as well as children and adults Primes T cells TI-1: Antibody response occurs in infants, as well as children and adults NO T cells needed Does NOT prime T cells High concentrations --> polyclonal B cell activation TI-2: Antibody response does not occur in infants and becomes more potent as a person gets older T cells are not needed, but cytokines from other cells augment TI-2 responses and induce class-switching Does not prime T cells Requires repeating epitopes, such as a repeating polysaccharide moiety on bacterial surface

Question 38

IgM

Answer 38

First to be made – does not require class switching Some IgM is always made, even after somatic hypermutation, but other types dominate at later times. Low affinity --> made before somatic hypermutation Forms pentamers that bind simultaneously to multivalent antigens (e.g., bacterial polysaccharides) – the co-operativity compensates for low affinity. Mainly found in blood Especially effective in activating complement. Blood infections must be dealt with fast or you die, and IgM

Question 39

IgG

Answer 39

Can diffuse easily out of blood into tissues Principal isotype in blood and extracellular fluid Efficiently opsonizes pathogens for engulfments by phagocytes Activates complement Always monomeric Maternal IgG is transported across the placenta directly into the bloodstream of the fetus Prevents adhesion of pathogens to tissues Neutralizing antibody

Question 40

IgA

Answer 40

Can diffuse easily out of blood into tissues Principal isotype in secretions Especially important for the gut and respiratory tracts Always monomeric Neutralizing antibody Not a good opsonin Not a good activator of complement Operates mainly on epithelial surfaces where complement and macrophages are not normally present Can form dimers IgA-secreting cells are found in connective tissue called the lamina propria (below basement membrane) Synthesis and secretion: Gut Respiratory epithelium Lactating breast Exocrine glands (salivary and tear) Role: protect external epithelial surfaces from infectious agents Secreted into breast milk and are transferred to the gut of the newborn infant (less diarrhea)

Question 41

IgE

Answer 41

Can diffuse easily out of blood into tissues Almost all is bound to high affinity receptors on mast cells Just beneath the skin and mucosa Along blood vessels in connective tissue Very low levels in blood or extracellular fluid Binding to IgE on mast cells induces release of power chemical mediators that cause coughing, sneezing, and vomiting. Expulsion of infectious agents. Always monomeric

Question 42

High-affinity IgG and IgA can neutralize bacterial exotoxins.

Answer 42

Exotoxins are secreted by pathogenic bacteria Endotoxins are released only upon death of the bacterium One exotoxin may kill an entire cell Consist of 2 protein chains IgG (inside the body) and IgA (on the other side of mucosal surfaces) --> principal neutralizing antibodies

Question 43

Vaccines

Answer 43

Cause you to make neutralizing antibodies to the receptor part of the exotoxin If the toxin consists of 2 peptide chains (Diptheria), its possible to immunize with just the receptor-binding chain Toxic chain destroyed by denaturation. Resulting vaccine is called a “toxoid Induce synthesis of protective antibodies, which are harmless Some insect and snake venoms will kill you before you can make your own antibodies. Horses are immunized to produce anti-venom antibodies (antivenins) for “passive immunization” of humans

Question 44

Active vs. Passive

Answer 44

In normal vaccination, we are induced to make our own antibodies. With antivenins, antibodies have been made previously by immunizing horses. Antivenins do their job for us and are then eliminated. We do not acquire any long-lasting immunity.

Question 45

Enveloped viruses

Answer 45

Viral nucleic acid is packed into a protein coat, surrounded by a membrane that can fuse with the host membrane 1.) Bind to receptor on cell surface 2.) Gain entry to the cytoplasm either by: Fusion of the virus envelope with the cell membrane Endocytosis, followed by fusion of the virus envelope to the surface of the endosome (more acidic environment that encloses virus) **Both dump the virus ion the cytoplasm of the cell

Question 46

Non-enveloped viruses

Answer 46

1. ) Virus attaches to a specific receptor on the target cell surface 2. ) Virus is internalized by endocytosis 3. ) Virus enters the cell cytoplasm by disrupting the endosome that encloses it.

Question 47

Antibodies can block the adherence of bacteria to host cells

Answer 47

Bacteria may cause disease by binding to the surface of sensitive cells. Involves: specific receptor-binding protein on the surface of the bacterium (adhesin) +specific receptor on the surface of the target cell Some pathogens enter cell via endocytosis or remain attached to cell surface (extracellular) Ex: gonorrhea: cell surface protein, pilin Allows bacterium to adhere to epithelial cells of urinary and reproductive tract

Question 48

Antibody:antigen complexes activate the classical pathway of complement

Answer 48

C1: 2 C1r, 2 C1s bound to 1 C1q Antibodies attach to surface of pathogen, binding to C1q C1q can bind to the Fc portion of either IgM or IgG antibodies C1q can't bind to a single antibody molecule Binding occurs when antibodies are bound to multiple sites on a cell surface (binding of 2 or more globular heads activates C1q) One Pentameric IgM or Two or more IgG molecules Complement cascade starts with activation of enzymatic activity in C1r

Question 49

Immune-complex removal

Answer 49

Formed tom binding to exotoxins, or also to debris from dead Immune complexes contain too few Fc portions of IgG molecules to allow them to be readily phagocytosed by macrophages (need have Fc receptors crosslinked in order to phagocytose a pathogen). Failure to remove immune complexes leads to deposition on the podocytes, resulting in damage to renal glomerulus and kidney failure.

Question 50

Immune-complex Removal Steps

Answer 50

1. ) Soluble immune complexes bind to C1q 2. ) Complement cascade of proteases is activated --> covalent binding of C4b and C3b to complex 3. ) Bound C4b and C3b bind to CR1 receptor on erythrocytes via bound C3b 4. ) Erythrocytes transport bound complex (antigen, antibody & complement) to the liver and spleen 5. ) Macrophages with CR1 and Fc receptors remove the complexes from the erythrocyte surface w/o destroying the cell

Question 51

Fc receptors

Answer 51

Cell-surface molecules that bind to the Fc portion of imunoglobulins Each recognizes Ig of one isotype Different accessory chaing (i.e. B cells, macrophages, neutrophils, etc.) bear Fc receptors for antibodies of different isotypes Isotype of antibody determines which accessory cell will be engaged Functions: Activation of accessory cells Expressed by dendritic cells --> enables them to ingest antigen:antibody complexes of present antigenic peptides to T cells

Question 52

Phagocytes

Answer 52

Activated by IgG that bind to specific Fc-gama receptors on the phagocyte surface Aggregation or multimerization of antibodies that occurs when they bind to multimeric antigens and multivalent antigenic particles (viruses and bacteria) Several antibody molecules, bound to the same surface, bind to multiple Fc receptors on the surface of the accessory chain Fc receptors cross-linking sends a signal to activate (or sometimes inhibit) the cell bearing it Bind with higher avidity than immunoglobulin monomers

Question 53

Important accessory chains in humoral response

Answer 53

Macrophages and neutrophils

Question 54

Comparisons

Answer 54

For Bacteria: Phagocytes are macrophages and neutrophils For large Parasites Phagocytes Fc-gamma and Fc-alpha trigger: Internalization of bacteria by phagocytosis Or externalization of internal vesicles (in mast cells) by exocytosis (depends on the cell) Fce receptors (specific for IgE) triggers only exocytosis

Question 55

NK cells

Answer 55

Have Fc receptors that recognize IgG bound on target cells (our OWN cells) Bound antibodies have Fc receptors that point away from the cell surface Express FC-gamma-III which binds to IgG1 and IgG3 Activated NK cell kills its target by the same mechanism that a cytotoxic CD8 T cell kills a target cell ADCC (antibody-dependent cell-mediated cytotoxicity) Allows NK cells, which have NO antigen specificity themselves, to kill cells

Question 56

Mast Cell

Answer 56

& basophils bind IgE antibody via the high-affinity Fce receptor (respond rapidly) Fc-gamma receptors also can be cross-linked, but Fc receptors are the most important. Mast cells are located in high concentrations near body surfaces – in places where infectious agents are most likely to enter. Mast cells, when activated, increase the flow of lymph – promoting activation of the adaptive immune response. Mast cells trigger muscular contraction, promoting expulsion of the pathogen Mast cell activation leads to recruitment and activation of eosinophils and basophils These three cell types are vital to defense against parasites. Blood-sucking ixodid ticks. A tick bite results in mast cell degranulation. Ticks won’t bite after IgE antibodies develop. Allergic responses to schistosomiasis in industrialized countries.

Question 57

Activated Mast Cells

Answer 57

Activated by: antibody bound to Fc receptors specific for: IgE (FceRI) IgG (Fc-gamma-RIII) Releases contents of its granules within seconds Releases stored histamine (causes increase in blood flow and vascular permeability) Synthesizes release of lipid mediators and cytokines (local inflammatory response) Influx of blood-borne cells Part of front-line host defense against pathogens that enter the body across epithelial surfaces

Question 58

FceRI

Answer 58

Binds to monomeric IgE antibodies with extremely high affinity (Fc portions bound to receptor and Fab portions facing outwards) Multivalent antigens cross-link the antibodies bound to FceRI and activate mast cells

Question 59

B cell activation by helper T cells

Answer 59

BCR: signals antigen has been bound and provides antigen as a peptide fragment to helper CD4 T cells Effector CD4 T cells: make cytokines B cell proliferation and differentiation Class switching from IgM Naive T cells require co-stimulaory signal from professional antigen presenting cells Naive B cells require accessory signals that come from helper T cells Pathogens that elicit B cell responses without T cell help: (antibodies they produce) Earlier response to pathogen Lower affinity Less functionally versatile