Humoral Immunity

Question 1

Immunoglobulin (antibody) molecules functions in the humoral immune response

Answer 1

1) antigen recognition
Membrane assocated antibody IgM is the receptor that B cells use to repscifically recognise antigens. The antibodies that B cells produce have the same specificities as the receptors on that B cell

2) Effector function
Soluable antobodies are secreted by antigen-activated B cells and plasma cells. Antibodies are the major effector molecules of humoral immunity. Different antibody types are specialized for different locations and functions

Question 2

Antibodies are defined by what

Answer 2

Isotype and idiotype

Question 3

Isotype

Answer 3

defined by the constant domain (ie IgG, IgM, IgE, ect) This domain determines the effector function of Ig molecule

Question 4

What domain determines the effector function of the Ig molecule

Answer 4

The constant domeain

ie isotype

Question 5

Idiotype

Answer 5

determined by the variable and hypervariable domains of the Ig molecule. The idiotype determines the specificity of the anitobody molecule

Question 6

What determines the specificity of the antibody molecule

Answer 6

hypervsariable and variable domains

ie idiotype

Question 7

B cell development occurs in two destinct stages

Answer 7

Antigen intependent phase

Antigen dependent phase

Question 8

Antigen independent phase of B cell development

Answer 8

occurs in bone marrow

Diverse B cell receptors generated (>10^9 specificities)

Test for. functional BCR (b cell receptor)

Select out self reactive cells

not dependent on specific interactions with other immune cells

Question 9

Antigen dependent phase of B cell development

Answer 9

Occurs in peripheral lymphoid tissues

Selecting and amplifying responses

avoid responding to peripheral selft antigen

increase affinity to antigen

select Ig subclass, which influcences effector function

generate memory cells for recell responses

critical role of CD4+ T helper cells

Question 10

Answer 10

Review of B lymphocyte maturation

Pro B cell expands. THen has pre B cell heavy chain. If failed receptor goes to apoptosis. If immature B cell IgM+ reacts to self antigen it will either receptor edit or apoptosis

Then goes to periphery

MAture B cell IgM+, IgD+ responds to antigen

it secretes antibody and establishes memory

Question 11

Answer 11

Overview of B cell responses in the periphery

Naive B cell recognises microbial antigen

T cell helps with antigen presentation and B cell is activated

Clonal expansion

Differentiation
You can wither have class switch or affinity maturation
Also make memory B cells IgG+

Question 12

Role of peripheral lymphoid organs in B cell development

Answer 12

1) Where B cells encounter concentrated antigen

2) Where B cells interact with othet immune cells
- T helper cells: critical for clonal B cell ploliferation and maturation
- Folicular dendritic calls and Tfh: mediate affinity maturation

3) Provides the appropriate microenvironment to influence effector function (through class switching)

Question 13

Answer 13

Peripheral lymphoid organ

Question 14

Parts of peripheral lymphoid organ

Answer 14

Primary follicles (B cell zone)- Where B cells encounter antigen

Paracortex (T cell zone)- Where T cells encounter antigen presenting cells (APCs)
(dendritic cells travel here when they have found antigen)

Secondary follicle (germinal center)- consequence of the interaction between B and CD4+ T cells

Question 15

How do B cells find antigen?

Answer 15

Antigen enters the nodes via the afferent lymphatics and is either trapped in the subcapsular sinus by specialized macrophages or is delivered to the follicles via conduits

Recirculating B cells migrate from the blood into the follicles of the lymph node

Upon Ag engagement via the B cell receptor, the B cells move towards ther paracortex (where the T cells hang out)

Without Ag engagement, the B cell leaves the node through the efferent lymph and recirculate between and through secondary lymphoid tissues

Question 16

How do B cells find antigen in my words and in picture

Answer 16

Antigen enters node in afferent lymph

Ag trapping cells in subcapsular sinus trap antigen

B cell follicle- Recirculatnig B cell migrates from blood to follicular mantle

Engages Ag via BCR, inducing interaction with primed T cells

Engages Ag via +CR2 and trandports it to FDC

If no Ag involvement, B cell leaves node in lymph

Question 17

Answer 17

Microbial Antigen Recognition

Microbe binds to antigen receptor (which has no signaling capability) However, Igalpha and Igbeta have ITAM which is a signaling pathway ans helps them to form signal cascade ultimately leading to transcroption

Question 18

Features of antigens that promote B cell responses

1) bound complement

Answer 18

B cell signaling is enhances by complement bound to antigen

When a pathogen is coated with C3d, engagement of the CD21/CD19 complex by C3d synergizes with signaling through the B cell receptor

Question 19

Features of antigens that promote B cell responses

2) pathogen multivalency

Answer 19

Highly multivalent antigens induce stronger B cell responses

Some highly multivalent angigens can induce T-cell independent responses

Question 20

Features of antigens that promote B cell responses

3) pathogen-associated molecular patterns (PAMPs)

Answer 20

Most pathogens contain PAMP that can activate PRR receptors (such as TLRs) expressed by B cells

Signaling through toll-like (and other) receptors synergizes with BCR signaling

These lignds acan also induce polyclonal activation of B cells

Question 21

What are the functional cconsequences of antigen binding to the B cell receptor ?

Answer 21

1) entry into the cell cycle
2) Low level IgM secretion (short lived Plasma cells)

3) Expression of molecules that are important in subsequent interactions with T helper cells
These include:
Co-stimulatory molecules
Cytokine receptors
Presentation og Ag on MHC II

4) Migration out of the follicle toward the T rich zone (CCR7 up arrow)

Question 22

What happens when CD4+ T cells respons to antigen

Answer 22

Antigen presentatio by DC to helper T cell

Activation of CD40 ligand, cytokine secretion

Entry into cell cycle (proliferation)

differentiation into effector T cells

• Secretion of cytokines and expression of surface molecules
• Migration towards B cell zone (lymphoid follicle)
• Enter circulation and migrate to the site of infection

Question 23

Overview of B and T cell interaction in the Lymph Node

Answer 23

B cells amd T cells are segregated in the lymph node

B cells encounter antigen in primary follicles
T cells encounter antigen presenting ccells in the T rich zone

Upon antigen recognition, both cell types modulate CCR7 expression and migrate towards eachother

STEPS
Antigen presentation:T cell activation
CCR7 down and CXCR5 up and migration of activated T cekks to edge of follicle
B cell presents antigen to activated helper T cells
Antigen uptake and precessing; B cell activation
B cells increase CCR7 and migration of activated B cells into folicle

Question 24

The specificity of the B/T cell interaction in mediated by MHC class II presentation of internalized antigen

Answer 24

B cell interacts with microbial protein antigen

Receptor mediated endocytosis of antigen happens

antigen processing and presentation happens on class II MHC peptide complex

T cell recognition of antigen (activted CD4+ T cell)

Question 25

B cell antigen presentation and the concept of linked T helper epitopes

Answer 25

MHC can only display proteins If you have a sigar specific B cell will do receptor-mediated antigen internalization, proteolysis, and presentation of peptides Will then display protein on cell and present it ot a protein specific T cell

Question 26

Conjugate vaccines

Answer 26

A technique for proviting T helper responses to antigens that do not have T cell epitopes Polysaccharide (PS): contains B cell epitopes but lacks T cell epitopes Tetanus Toxoid (TT): contains both B and T cell epitopes Immunize with PS B cells PS specific Th cells: NONE IgG response: NONE Immunize with TT-PS conjugate B cells: PS-specific and TT specific TH cells: TT specific IgG response: anti PS IgG and anti-TT IgG

Question 27

Key components of T cell help

Answer 27

Activation of T cell; expression of CD40 ligand, cytokine secretion Activation of B cell and by cytokines and CD40 ligation B cell proliferation and differentiation The specificity of this interaction is maintained by: 1) Cognate interaction mediated by cell-cell contacts 2) Interaction occurs in a specific location at the boundary of the T-rich zone and the primary B cell follicle X-linked Hyper IgM syndrome is due to a mutation in CD40L; B cells in these patients do not class switch

Question 28

What are the consequences of the T:B cell interaction

Answer 28

1) Clonal expansion of B cells 2) Establishment of germinal centers 3) Class switching 4) Affinity maturation 5) Differentiation to memory cells In general, all of these events are predicted on B cells recieving help from CD4+ T cells

Question 29

The germinal center reaction

Answer 29

Upon activation by CD4+ T cells, B cells begin to proliferate and enter what is known as secondary follicles Secondary follicles are home to germinal center Each follicle is mainly clonal, and the germinal center contains two regions, the light zone and the dark zone In the dark zone, B cells undergo rapid cell division and turn on the machinery that leads to: 1) somatic hypermutation of the variable regions of the B cell receptor 2) Class switching The light zone of the germinal center is rich with follicular dendritic cells (FDCs) and CD4+ T cells (TFH cells). B cell survival is dependent on interaction with FDC-bound Ag and presentaion of Ag to T cells

Question 30

Antibody affinity selection

Answer 30

Affinity selection is the process that leads to increqased affinity of antibodies for a particular antigen as a T cell dependent humoral immune response progress Mutations and affinities were measured after vaccinations. Shaded regions are CDRs (complementary determining regions) The mutations are found here This shows that the antibodies are getting mutations to increase binding

Question 31

Affinity selection is mediated by 3 factors (affinity maturation)

Answer 31

1) Rapid division After interacting with CD4+ T cells, B cells rapidly proliferate (divide every 6-12 hours) 2) high mutation rate Expression of the protein AID ( activation induces deaminase) leads to hypermutation of the V domain of the BCR (1 error per 1000 bases per cell division) 3) Selection B cells compete for binding of limiting amounts of antigen displayed by folicular dendritic cells. Only cells taht successfully bund to antigen and then recieve help from Tfh are selected sa

Question 32

Affinity selection: Evolution in a germinal center

Answer 32

Antigen is taken up by FDCs in the form of immune complexes and held in a non-degraded form for month after an infection. FDCs + antigen + Tfh + CD40L provide survival signal to B cells Taken together, this process selects for mutations that lead to stronger binding of antibody to antigen

Question 33

B cell class switching

Answer 33

Class switching- a biological mechanism that changes a B Cells production of antibody from one class (isotype) to another Different antibody types have different effector functions Class switching is mediated by cytokine environment cytokines are largely produced by Th cells. Different Th subsets produce different cytokines and promote switching to different isotypes Th1 produces INF-gamma which mediates class swithcing to IgG Th2 produces IL-4, IL-5, IL-13 which mediates class switching to IgE

Question 34

Th1 produces ____ which mediates class switching to \_\_\_

Answer 34

INF-gamma IgG

Question 35

Th2 produces ____ which mediates class switching to \_\_\_

Answer 35

IL-4, IL-5, IL-13

Question 36

There are 5 different antibody classes based on different

Answer 36

heavy chains

Question 37

IgM and IgD

Answer 37

Expressed on naive B cells. Both are expressed as membrane receptors, but only IgM is expressed as soluable antibody

Question 38

IgG, IgE, and IgA

Answer 38

Expressed on ACTIVATED or MEMORY B CELLS that have undergone class switching. All 3 molecules can be expressed as membrane receptors or as soluable antibody

Question 39

How do cytokines mediate class switching?

Answer 39

1) Cytokines induce transcription from specific promoters. (For example, IL-4 induces transcription from Ie) 2) Transcription triggers a recombination events that result in the deletion of intervening constant (C) genes 3) The resulting DNA can be transcribed and translated 4) This process results in an antibody with the same specificity, but a switched constant domain

Question 40

IgM effector function

Answer 40

produced early upon B cell activation activates the classical complement pathway

Question 41

IgG effector function

Answer 41

the major Ig isotype in the blood Transferred across the placenta Activated the classical complement pathway mediated neutralization and opsonization of pathogens mediated antibody-dependent cellular cytotoxicity

Question 42

IgA isotype

Answer 42

Produced in mucosal tissues and secreted into the lumens of the GI and respiratory tracts Mediates pathogen neutralization Can be transferred to infants via breast milk

Question 43

IgE effector function

Answer 43

defense against helminths involved in allergic responses

Question 44

Effector function os antibodies

Answer 44

Some of them are mediated by Fc receptors

Question 45

Memory B cells

Answer 45

Generated in germinal centers therefore we only have humoral memory to T dependent antigens Circulate through lymphoid organs Express the affinity matured BCR on their surface Typically isotype switched (eg IgG+ or IgA+) Very long lived persistence of memory B cells after an immune response ensures that we have increased numbers of B cells specific for the antigen and ready to rapidly expand and differentiate into plasma cells and secrete antibody upon reexposure to antigen Provide the basis for secondary GC reactions

Question 46

Plasma cells

Answer 46

arise predominantly from germinal ceners (in primary responses) or from memory B cells (in secondary responses) Are very long lived (several months or longer) Often home to bone marrow, gut, lactating mammary gland Are predominantly isotype switched

Question 47

Features of primary and secondary antibody responses to a T DEPENDENT ANTIGEN

Answer 47

Notice that hte first response takes much longer Antibody levels at peak are also much higher in second exposure In primary response, you make IgM and IgG and in secondary response we only see IgG

Question 48

Do you have IgM respsonse with a polysaccharide T independent response?

Answer 48

YES

Question 49

Some antigens can provoke B cells responses without the requirement for T help

Answer 49

Question 50

With T independent antigen first and second response

Answer 50

ARE THE SAME

Question 51

Which of the following events does NOT occur in the germinal centers somatic mutation of Ig V genes Ig gene V(D)G recombination B cell proliferation T folicular helper cell iteractions with B cells generation of memory B cells

Answer 51

Ig gene V(D)J recombination this is an early event that happens in the bone marrow

Question 52

Previously healthy young european woman who had not been immunized against measels virus became infected with the virus while traveling to disneyland developed a rash, high fever, cough, and runny nose, and then fully recovered after 4 days. A blood test performed 2 months later, after her return to europe, showed the presence of high affinity IgG antibodies specific for measels virus antigens. Which of the following proteins was NOT necessary for the production of these high affinity IgG antibodies? CD40 ligand Activation induced deaminase Class II MHC CXCR5 Fc gamma receptor

Answer 52

Fc Gamma receptor- helps regulate but not produce CD40 ligand- expressed on T cells. important for class switching activation induced deaminase (AID) - important in class switch and affinity maturation Class II MHC- B and T cells come together CXCR5- chemokine to go to zone

Question 53

Distinct B cell subsets mediate different types of antibody responses

Answer 53