Adaptive Inmunity

Question 1

Describe the basic structure of an antibody molecule

Answer 1

Symmetrical
Two heavy chains
Two light chains
Connected by disulphides bridges
The hinge region allows flexibility of the Fab- from parallel to right angles between arms
Fc region bound by Fc receptors made only from heavy chains
Fab is made from V(L), V(C), V(H), V(H1)
The variable regions comprise the antibody binding sites

Question 2

How do B cells recognise antigens?

Answer 2

Surface immunoglobulins
CD79 co receptors signal inside the cell once antigens have bound
Ig recognise antigens with their variable regions of heavy and light chains- hyper variable regions (complementarity-determining regions; CVR)

Question 3

How do T cells recognise antigens?

Answer 3

TcR is surrounded by CD3, and itself is an isolated, membrane-bound Fab region
Surface bound T-cell receptors (alpha-beta or gamma-delta)
Resembles the Fab region of an immunoglobulin
CD3 complex signals inside the cell when the peptide/MHC binding site binds a cell surface antigen
Has either a CD4 (T cell helper) or CH8 (T cell cytotoxic)
The recognise small peptides in the groove of MHC molecules

Question 4

Describe the thymus

Answer 4

Two main cellular elements
-haemopoietic BM-derived precursors
-stromal cells that provide the niche and building block
Capsule, Cortex and medulla (inner)
Blood vessel enters in the medulla with the medullary epithelium then there is the cortical epithelium with dendritic cells at the boundary, the subcapsular epithelium and then the capsule
The thymus involuted with age- T cells produced decrease

Question 5

Describe the stages of T cell development

Answer 5

Double negative- CD4-CD8-
In blood stream, migrate through the cortex towards the capsule
Double positive- CD4+CD8+
Migrate through the cortex to the medulla
CD4+ single positive or CD8+ single positive
Found in the medulla

Question 6

Describe the first checkpoint in T cell development

Answer 6

Maturation from double negative to double positive

• commitment to the T cell lineage (precursor needs identity so epithelial cells give notch signal, forming alpha,beta Tc instead of DC, NK)
• Cellular expansion (controlled by IL-2,7, 15 or sc factor)
• Rearrangement of genes encoding the T cell receptor- VDJ recombination of the beta chain

Question 7

How are T cells with fully rearranged TcR-beta chain genes selected?

Answer 7

Pre-T cell receptor complex
Expressed only on the double negative thymocytes

Consists of TCR-beta protein, CD3 and pre-Talpha

Assembly of this pre-T cell receptor signals for the cell to stop further beta rearrangement (allelic exclusion), CD4, CD8 expression, TCR-alpha rearrangement and cellular expansion

Question 8

Describe the need for positive and negative selection of TCRs

Answer 8

Genetic rearrangement is random so need to expand useful TCRs, recognise peptide-MHC of low affinity and avidity and remove any harmful TCRs, recognise peptide/MHC at high affinity and acidity via positive and negative selection

Question 9

Describe positive and negative selection

Answer 9

Affinity model- low affinity give positive selection
High affinity gives negative section
Avidity model- low avidity gives positive selection
High avidity give negative selection
Negative selection occurs via apoptosis
Positive selection happens in the cortex- there is a random binding of the TCR CD4 or CD8 to the MHC2 or MHC1 complex respectively on the thymic epithelium giving a single positive cell
Negative selection happens in the medulla- high affinity/avidity➡️apoptosis

Question 10

How are antigens presented on dentritic cells?

Answer 10

Expression on MHCs via the endogenous pathway for MHC1 expression- cytosolic derived proteins degraded and the peptide added to MHC1 in the RER and then trafficked to the surface
Exogenous expression for MHC2 as the externally derived protein meets MHC2 on its way to the surface (CLIP prevents other proteins binding until then)
Cross presentation of exogenous proteins will end up in the class1 pathway for viral protein presentation for other cells

Question 11

How dendritic cells mature?

Answer 11

Immature peripheral DCs have a variety of PRRs that are activated by PAMPs➡️ TLR signalling induces CCR7 and enhances antigen processing➡️ migrate into lymphoid tissues and increase expression of MHC and co-stimulatory molecules
Mature dendritic cells enter the lymph node form infects tissue and can transfer antigens to resident dendritic cells that can stimulate T cells

Question 12

How are T cells activated?

Answer 12

T cells enter the lymph node via the high endothelial venules and migrate into the T cell areas
Dendritic cells migrate there too and the T cells wonder over the DCs and if they are activated proliferate and differentiate before they can exit the lymph node

Question 13

Describe the molecular interactions between dendritic cells and T cells

Answer 13

MHC2-TCR + CD4- signal 1 Activation
CD28-CD80 CD86- signal 2 survival
Cytokines- signal 3 differentiation

Question 14

What is the importance of IL2 in driving T cell proliferation?

Answer 14

Resting T cells express a low affinity IL2 receptor (beta, gamma chains only)
Activated T cells express a high affinity IL2 receptor (with the alpha chain) and secrets IL2
Binding of IL2 sing aka did the T cells to enter the cell cycle so induces proliferation

Question 15

Name the different types of effector T cell subsets

Answer 15

CD8 cytotoxic T cells- kills virus infected cells
CD4 Th1 cells- activate infected macrophages and help B cells for antibody production
CD4 Th2 cells- provide help to B cells for Ab production especially switching to IgE (helminths)
CD4 Th17 cell- enhance neutrophil response- promote barrier integrity
Tfh cells- isotype switching
CD4 regulatory T cells- suppress T cell responses

Question 16

Describe the differentiation and function of CD4+ Th2 cells

Answer 16

Stimulated by IL4 
Secrete IL4,5,13
TF is GATA3
Promotes resistance to large extracellular helminth parasites
IL4 promotes class switch to IgE
Drives macrophage activation

Question 17

Describe the differentiation and function of CD4+ Th17 cells

Answer 17

Differentiation stimulated by IL-1beta, IL6, IL21 and maintained by IL23
TF is ROR-gamma-t
Secrets IL17
Protects against fungal infection and some bacteria
Increases neutrophil recruitment
Controls epithelial barrier function

Question 18

Describe the differentiation and function of CD4+ T follicular helper cells

Answer 18

Differentiate by IL21
TF is Bcl6
Express the CXCR5 chemokine receptor to allow migration towards B cell areas
Act within the second lymphoid compartment to help B cells

Question 19

Describe the differentiation and function of CD4+ T regulatory cells

Answer 19

nTreg cells derived from self-binding T cells
pTreg cells derived from any peripheral immune response
Differentiate in the presence of TGF-beta and retinoic acid
TF- FoxP3
Restrict other immune responses and T cell proliferation
Block signal 2 for autoimmune T cells causing them to apoptose and can inhibit tumour immunity

Question 20

Name some B cell markers

Answer 20

CD20
CD19
CD86
CD40

Question 21

What are the B cell populations?

Answer 21

B2Foll- follicular B cells
T dependent response such as proteins
Broad repertoire- includes protein
Found in spleen and lymph nodes
IgM+, IgD+, some CD21, high CD23
B2MZ- marginal zone B cells 
Naive and memory cells
T-independent- narrow repertoire
Spleen- few cells soon after birth 
IgM+, low IgD, high B220, high CD21, low CD23

Question 22

Describe the life a B cell

Answer 22

Generation in bone marrow- B cell precursor rearranges its immunoglobulin genes
Negative selection in bone marrow- immature B cell bound to self cell surface receptor removed from repertoire
B cells migrate to the peripheral lymphoid organs- mature B cell bound to the foreign antigen is activated
Antibody secreted and memory cells in bone marrow and lymphoid tissue- active B cells give rise to plasma cells and memory cells

Question 23

Describe B cell development

Answer 23

Develop on stromal surface - different stromal cells have different effects during development eg. CXCL12 expressing cells and IL7 expressing cells
Defined by Ig rearrangement
Heavy chain first- germline in stem cell➡️ D-J rearranging in early pro-B cell➡️ V-DJ rearranging in the late pro-B cell➡️ VDJ rearranged, mau chain transiently at the surface as part of pre-B cell receptor in large pre-B cells- mostly intracellular
Then light chain-
V-J rearrangement in and intracellular mau chain in small pre-B cell
VJ rearranged and IgM expressed on cell surface
VJ rearranged and IgD and IgM made from alternatively spliced H Cain transcripts
Rearrangement happen on one chromosome and if it is unsuccessful it is tried on the second- allelic exclusion

Question 24

What happens to prevent autoimmunity from B cells?

Answer 24

Multivalvent the self molecule➡️ clonal deletion or receptor editing➡️ apoptosis
Soluble self molecule➡️ migrates to periphery and become and Anergic B cell
Low-affinity noncross-linking self molecule➡️ migrate to the periphery and becomes a clonally ignorant nature B cell
No self reaction➡️ migrates to the periphery and becomes a mature B cell

Question 25

Describe plasma cells

Answer 25

Low surface Ig No MHC2 High Ig secretion rate No growth, somatic hypermutation or isotype switching because they are terminally differentiated

Question 26

What are three pathways to generating antibody?

Answer 26

1. B1 cells can make natural antibody without stimulus to a limited repertoire of antigens and in a T-independent manner 2. Extra follicular antibody- red pulp in spleen, medullary cords of lymph nodes responses may be T dependent or independent depending on antigen- Ab affinity usually low 3. Germinal centres are structure in which plasma cells that secrete high-affinity antibody and memory B cells develop and need T cells

Question 27

Describe briefly T-independent

Answer 27

Age is encountered in the periphery or in the MZ➡️ Ag-activated B cells transit via T zone➡️ extra follicular plasmablast growth resulting in rapid ab production High conc of TI-1 antigen (LPS) ➡️polyclonal B-cell activation, non-specific antibody response Low conc of TI-1 antigen➡️ TI-1 antigen specific antibody response TI-2 antigens alone can signal to B cells to produce IgM antibody

Question 28

Briefly describe the generation of primary T-dependent response

Answer 28

Naive T cells are primed by cognate integration with dendritic cells➡️ Virgin or memory B cells, on binding ag, move to T zone ⬆️CCR7 ➡️ Germinal centres gradually produce persistent high affinity ab and memory B cells➡️ ➡️cognate interaction of primed T- and activated B cells in the T zone➡️ extra follicular plasmablast growth resulting in rapid ab production in the medulla

Question 29

What is the role of the Tfh cell in the T-dependent response?

Answer 29

To be primed they recognise peptide/MHC2 by DC To positively select B cells they recognise peptide/MHC2 by B cell- signal through cognate interaction Essential additional molecules- CD40 for B cells, CD40L for T cells, ICOSL for B cells and ICOS for T cells Cytokines (IL4,6,21 and IFN-gamma) help reinforce cognate interaction

Question 30

Describe T cell help for B cell activation in the TD response

Answer 30

Tfh cell and B cell interact through TCR (+CD4)-peptide/MHC2, CD28-B7, CD40L-CD40 After antigen uptake, processing and presentation it up regulates B7 which binds to CD28➡️ CD28 transmits signals to the Th cell that include the stabilisation of mRNA for cytokines and ligands Th cell induced to express CD40L (and releases cytokines) which binds to CD40 to give a strong activation signal

Question 31

What is the importance for the germinal centres?

Answer 31

Long-lived plasma cell and memory B cells are derived there | Class switch to IgG or IgA- requires further Ig gen recombination that needs to be controlled

Question 32

Names cells found in the germinal centre

Answer 32

Centroblasts- proliferating GC B cells that undergo somatic hypermutation- dark zone Centrocytes- GC B cells that have undergone affinity maturation and are out of the cell cycle- light zone Follicular dendritic cells- they have native antigen bound to their surface and the centrocytes compete to bind the antigen- light zone Tfh cells- GC T cells any give out survive signals to centrocytes aches gene come out of the dark zone- light zone

Question 33

Name some important molecules involved in the Germinal centre responses

Answer 33

Activation induced cytidine deaminase (AID) essential for class switch recombination and IgV hypermutation BCL-6 master transcription factor for GC B cell commitment and generation of Tfh cells CD40 on B cells and CD40L on T cells IL21 for Thf cell development Blimp1 is a transcription factor required for the plasma cell programme

Question 34

After initial priming what are the two actions B cells can take?

Answer 34

1. Extra follicular responses where B cells differentiate to become IgM an IgG plasma cells moderate affinity antibody 2. Enter germinal centre response that gives rise to plasma cells producing high affinity, class switched antibody and memory cells

Question 35

Describe the germinal centre reaction

Answer 35

B cells entering the GC response form rapidly dividing centric lasts in the dark zone- each centroblast undergoes random point mutations within its IgV region genes These then exit the cell cycle and becomes centrocytes then competes for he antigen held in the form of immune complexes in follicular dendritic cells in the GC light zone Those centrocytes that have mutated to high affinity antigen receptors, compete most successfully and can the seek out antigen specific Th cells for cognate interaction in order to receive rescue signals- the rest die Positively selected centrocytes than exit the GC either as plasma cells or as memory cells

Question 36

Name the different antibody classes

Answer 36

``` IgG IgM- low affinity pentamer IgD- membrane bound IgA- mucous membrane dimer IgE- mast cells ```

Question 37

What are the functions of antibodies?

Answer 37

Neutralisation- virus entry, bacterial adhesion, toxins Complement activation- classical pathway Opsonisation- bind Fc receptors on phagocytes Type1 hypersensitivity- Multivalvent antigen cross links bound IgE antibody causing granule release from mast cells Antibody-dependent cellular cytotoxicity- bind target cell bind NK cells with Fc receptors (CD16) target cell dies Immune complex formation- small ag-ab completes form in circulation ➡️ complement activation- C3b ➡️ CR1 receptors on RBCs bind the C3b ➡️ phagocytes in spleen and liver remove the complexes from the RBC surface

Question 38

Describe the action of IgA

Answer 38

Transported through the gut epithelium by the polymeric Ig receptor Binds to the layer of mucous and neutralises pathogens and their toxins Is also in breast milk and is transferred into the child

Question 39

Describe IgG in child bearing

Answer 39

Can be transported across the placenta by TcRn IgM are John t transferred Last about 6-8 weeks

Question 40

How are anti-rhesus antibodies significant in a child bearing?

Answer 40

If mother is rhesus negative and child is rhesus positive blood can be mixed during delivery and TD mother will produces antibodies against the the rhesus possible blood so if the second child is also positive the anti-Rh antibodies will cross the placenta and attack the fetal RBCs