Immunity: The Humoral response

Question 1

Describe B cells

Answer 1

B cells secrete antibodies which bind to foreign antigens, allowing phagocytes to recognise and destroy them.

Each B-cell expresses 1 type of epitope antibody specific to 1 antigen. B cell receptors can also directly bind to a comp antigen. Microbes can have diff surface antigens, so each antigen type activates and binds diff B cells.

B cells can do phagocytosis, antigen presentation, and load antigens on MHCs to display to T cells

Question 2

Describe T cells

Answer 2

T cells produced in the bone marrow mature in the thymus. The thymus controls central tolerance- T cells differentiate into CD8+ or CD4+, and any T cells w self antigens are destroyed to avoid AI disease.

CD8+ =cytotoxic T killer cells. When activated, they destroy cells w nonself antigens on an MHC protein.

CD4+ are helper T cells (Th). They release cytokines that induce Bcells to differentiate into plasma cells which secrete antibodies.

Treg is a type of Th cell. It inhibits other T cell function, controlling immune response and tolerance

T cells each have 1 specific antigen receptor on its surface to bind to a comp antigen and get activated.

Question 3

How are T cells divided and differentiated?

Answer 3

ProT (immature T precursors) undergo selection in the thymus

As the T-cell matures, it will acquire the Alpha beta or the Gamma Delta type T-cell receptor.

Only the αβ cell makes a CD4 & CD8 co receptor, so they are CD4+ and CD8+. γδ cells are CD4- and CD8-.

αβ cells differentiate into either CD4+ or CD8+

After a pathogen invades, CD4+ T cells differentiate into either Th1, Th2, Th17 and Treg cells

Question 4

What is the endogenous pathway of T cell priming?

Answer 4

Endogenous antigen pathway leads to CTL priming

Cells are infected by a pathogen/virus.

Viral antigens are presented with MHC Class I molecule on cell surface.

CD8+ T cell recognises the antigen via TCReceptors, becomes cytotoxic and starts making cytokines.

CTL proliferates to make memory cells y mas CTL cells looking for comp APCs. CTLs destroy infected cells.

Question 5

Describe the exogenous pathway of T cell priming

Answer 5

Exogenous antigen pathway leads to Th cell priming.

Macrophage engulfs a pathogen into a vesicle. The pathogenic antigen is processed into small peptides

Antigen is presented with MHC Class II on cell surface. T helper cell (Th) recognises the APC.

CD4+ T cell becomes primed. Now it either activates B cells by releasing cytokines (Th2) or macrophages (Th1)

Question 6

Describe T cell INdependent B cell activation

Answer 6

A pathogen containing multiple identical antigen epitopes can STRONGLY activate B-cells (rare).

Activation initiates proliferation of B cells and B memory cells. B cells are converted to plasma cells which make pentameric IgM

This pathway is restricted to specific antigens and isn’t checked for tolerance- can lead to problems eg TSS

Question 7

Describe T cell dependent B cell activation

Answer 7

B cells bind to a comp antigen on an APC to become APCs themselves. Chemokines are released which attract Th cells and activate B cells. B cells bind w a comp CD4+ cell.

The B cell becomes activated and proliferates into: B effector cells which produce plasma cells⇢release IgM. B memory cells

Ab production needs co-stimulation with a Th cell that was already screened against self, so auto-immunity is avoided

Question 8

Draw, label and describe an antibody structure

Answer 8

4 pp chains in a Y shape, held by non-covalent interactions and disulphide crosslinks between cysteine residues.

Fab binds the antigen. Fc binds to receptors on phagocytes, activates complement. Fc gene section determines which Ig isotype is produced.

Hay 2 identical light chains, kappa (κ) or lambda (λ), never both. Also hay 2 identical heavy chains.

Carbs help assembly and binding to cells.

Question 9

What are the different types of IgM antibody structures?

Answer 9

IgM monomer has a long heavy chain which binds to the B cell and stick out of it.

This monomer can form a pentameter structure which is secreted & highly antigenic. It contains a J chain- a polypep involved in pentamer polymerisation

Question 10

Describe IgG antibodies

Answer 10

IgG is the major class overall. Only IgG can cross the placenta to protect the fetus. V good at activating complement via classical pathway (removing the pathogen)

The Fc fragment at the bottom is an opsonin, which means it can induce phagocytosis: it can bring in infected organisms to immune cells like neutrophils /macrophages to be destroyed. This is bc the Fc region is recognised by Fc-receptors on immune cells

IgG can be present for long periods in the serum so may indicate past exposure and not current infection.

Question 11

What is the function of IgM antibodies?

Answer 11

IgM is better at activating compliment via classical pathway bc its pentameric structure means it has more antigen binding sites. IgM binds higher number of epitopes, enhancing phagocytosis

Presence of specific IgM antibodies to an antigen indicates a recent primary response to that antigen- current infection

Question 12

Draw a graph to describe the primary response to infection

Answer 12

The “switch” on the graph represents the isotype switching- this is the switch from making IgM to IgG antibodies.

The adaptive response to new infections is SLOW and SPECIFIC w long lasting memory.EMORY

Question 13

How does isotope switching work?

Answer 13

Ig heavy chain genes are in a specific order.

T cells produce cytokines, which will interact w the B cells. The interaction will cause chem reactions w in the cell that synthesise specific DNA cutting enzymes. This rearranges the Fc region, so the class of antibodies produced changes. The Fab region holding antigen specificity does not change

Question 14

What is the secondary response?

Answer 14

The i.response is faster bc memory cells recognise the antigen and differentiate straight away to produce antibodies. ⇡ IgG is produced for a longer time period.

The organism is destroyed so rapidly the person is unaware of any symptoms and is now immune to the disease.

Question 15

How does antibody-antigen interaction work?

Answer 15

Hay non-covalent interactions entre antibodies and ag: Electrostatic, hydrophobic, vdw forces, H bonds

The antibody b.site must being sterically and chemically comp w a site on the surface of the antigen.

A single antigen can have many possible binding sites (epitopes)

Question 16

How are natural immune responses polyclonal?

Answer 16

More than one clone of B-cells is generated and more than one Ig is synthesised

This is bc hay multiple antigens on organism, multiple epitopes on each antigen. Therefore more than 1 Ig may recognise the same epitope

Question 17

How does antibody fight infection?

Answer 17

By coating and neutralising a pathogen, eg if a virus is coated with Ab it can’t bind to its receptors on the cell surface

By activating complement, which can then form holes in a bacterial cell membrane

By opsonisation: phagocytes bind to pathogens coated with Ab and phagocytose them due to Fc receptors

Question 18

Describe IgA antibodies

Answer 18

Secretory IgA -most abundant in external bodily secretions (milk, sweat, tears etc)

1st line of protection at external surfaces. It binds Fc receptor on immune cells which initiates inflammatory reactions. Circulatory IgA can’t activate complement.

Secretory component protects IgA from degradation, so it can work in harsh environments (eg, GI tract)

Question 19

Describe IgE antibodies

Answer 19

IgE binds to specific Fc-receptor on mast cells/basophils, releasing histamine during allergic reactions. Over response can cause anaphylactic shock

Activates eosinophils via Fc receptor binding in response to parasitic worms

Low conc in circulation but raised in allergic patients and large parasitic infections

Question 20

Describe IgD antibodies

Answer 20

extremely low conc in circulation

Mainly found on surface of B cells as antigen-binding B cell receptor

Does not bind complement. Probably helps B cell activation

Question 21

What is the difference between BCRs and TCRs?

Answer 21

B cell receptors can recognise and bond to an antigen directly.

T cell receptors need cells to process and present antigens on their surface in the form of MHCs

Question 22

What is MHC class I?

Answer 22

MHC class I: present in all nucleated cells, inc APCs. The cells continuously break down intracellular peptides from the cytosol and present it on MHC I.

This means infected or cancerous cells display mutated peptides. These r recognised by CD8+ CTLs

Exception: Some APCs like dendritic cells present engulfed peptides normally in MHC class II, or MHC class I. This is cross presentation

Question 23

What is MHC class II?

Answer 23

Present on professional APCs only (mononuclear phagocytes, dendritic cells). APCs have class I and II MHCs.

APCs engulf extracellular pathogens. Pathogenic peptides are processed in vesicles. Peptides bind w MHC class II sent from the ER. This is presented on the APC to be recognised by CD4+ Th cells.

Exception: autophagy, where the cell degrades part of itself to remove misfolded/damaged protein. Although the peptides come from within the cell, they’re displayed on MHC class II instead of I.

Question 24

Describe the structure of MHC class I

Answer 24

MHC class I consists of 2 protein subunits, the alpha and the beta-2-microglobulin chains. A transmembrane region anchors the molecule on the cell membrane.

The alpha chain is larger and consists of alpha 1, 2 and 3 subunits. Alpha 1 and 2 form the peptide-binding cleft which presents the antigen. Disulphide bonds in alpha-2, alpha-3 and B-2 domains stabilises the protein.

The b-2-microglobulin interacts w alpha chain via other forces too

Question 25

Describe the process of peptides being loaded onto MHC class I

Answer 25

Old proteins undergoing degradation are tagged by ubiquitination. This tells the proteasome to chop the proteins into peptides. Some peptides are broken down into aa.s to be recycled.

Peptides are trafficked into the ER where it’s loaded onto MHC class I. It’s presented on the cell surface.

If normal peptides presented, cytotoxic T cells won’t recognise it, the cell will carry on as usual.

Infected/cancerous cells display mutated or viral proteins. CTL recognise and destroy these

Question 26

Describe the structure of MHC class II molecules

Answer 26

MHC class II molecules consist of 2 symmetrical subunits, the alpha and beta chain.

Both alpha and beta chains have 2 domains stabilised by disulphide bonds.

The alpha 1 and beta 1 subunit form the peptide binding cleft. Both chains have a transmembrane region anchor.

Question 27

What is the difference between alpha beta T-cell receptors and beta Gamma T cell receptors?

Answer 27

Only αβ cells will have CD4 and CD8 co-receptors, but lose 1 of them as they further differentiate

Beta Gamma is mostly found in gut mucosa. Unlike alpha beta cells, they don’t need MHC presentation to work

Question 28

Draw and label and alpha beta and Delta Gamma T-cell receptor.

Answer 28

γδ TCRs similar to αβ TCRs, but different chains

Question 29

What must occur in order to activate a T cell?

Answer 29

3 signals need to be present. This avoids premature T cell activation, avoiding healthy tissue damage.

1st signal: binding of the TCR to peptides presented on MHCs and the binding of CD4/CD8 to the MHC.

2nd signal: binding of APC and TCreceptors creates a co-stimulation signal. eg, the CD86/CD80 receptor, or the CD28 receptor. This confirms the T cell identity

3rd signal: Cytokines. CD4+ Th= large cytokine source

Question 30

Describe the Th1-mediated immune response

Answer 30

Th1 fights viral and bacterial intracellular infections. Eliminates cancerous cells. Produces proinflammatory responses

Stimulates B cells to produce IgG for opsonisation. Tagged bacteria by IgG is phagocytosed by other wbcs

Main cytokine to trigger its stimulation=polarizing IL 12.

The effector produced by Th1 is interferon gamma (IFNγ). This increases the macrophage’s capacity to engulf infected cells, produce MHC molecules to present peptides

Question 31

What happens if the Th1 mediated response is not regulated?

Answer 31

Th1-mediated disease can occur:

Granulomas, MS, Some autoimmune disease like T1 diabetes.

Question 32

Outline the process of phagocytosis

Answer 32

Tasting: phagocytes assess the identity of the foreign body. Pathogens tagged w Ig are identified faster.

Pseudopods by phagocytic cells form. If these touch, the pathogen is engulfed to form a phagosome.

A lysosome fuses w the phagosome which needs IFNγ signals from Th1 cells

Pathogen is digested by oxidative/non oxidative chemicals in the lysosome and presented on MHC class II. Th1 binds to the MHC class II and releases IFNγ. This enables the lysosome to fuse w the phagosome and activates the macrophage to send signals.

Question 33

What happens once a phagocyte/macrophage is activated by Th1 cells?

Answer 33

Macrophages increase lytic chemical/enzyme production to kill phagocytosed pathogens

They secrete cytokines IFNγ, TNFα, IL-2. These cause inflammation, recruit neutrophils & ⇡ phagocytosis

They send tissue repair signals eg Platelet-derived growth factor (PDGF) to regulate cell growth, TGF-β for collagen synthesis, fibroblast growth factor (FGF) for angiogenesis (making new blood vessels)

Question 34

Describe Th2 mediated response

Answer 34

Th2 activates humoral responses against toxins, extracellular parasites & bacteria. Main cytokine to trigger its stimulation: IL-4

Main effector cytokine: IL-4 (produces IgG2, IgG4, IgE, mast cell degranulation)

IL-5 (eosinophil activation)

IL-13 (prepares organs to expel helminths, anti inflammatory, activates tissue repair)

If Th2 isn’t controlled it can lead to allergies & asthma

Question 35

What is Antibody-dependent cellular cytoxicity (ADCC)?

Answer 35

A process where antibodies produced by B cells tag pathogens by binding its variable region to the pathogen surface antigens.

Diff immune cells recognise and bind to the constant region of the antibody, releasing ROS, RNS and lytic proteins to kill the pathogen/ parasite

Question 36

Which cells are capable of ADCC?

Answer 36

Macrophages activated by IFNγ bind to IgG/IgA antibodies. NO is the cytotoxic weapon.

Neutrophils bind to IgG and release H202.

Eosinophiles activated by IL5 bind to IgE and release several proteins like MBP or ECP.

Mast cells bind to IgG, IGA and IgE. They release ROS and proteases

Question 37

Describe the first mechanism of CTLs once activated.

Answer 37

Secretion of cytokines e.g. TNF-α, IFN-γ.

TNF-α is produced by CTLs, macrophages and Th1. It orchestrates the inflammatory response. IFN-γ is produced by CTLs, Th1 and NK cells. It activates macrophages, stimulates neutrophils and NK cells.

Question 38

Describe the 2nd mechanism of CTLs once activated.

Answer 38

They release cytotoxic granules in target cell direction to avoid killing healthy cells. They do this by releasing the granules along the immune synapse (where the TCR binds to the MHC molecules.)

CTLs have a serial killing ability – they release granules to kill 1 cell, then move on to new target and kill again

Question 39

What chemicals are in cytotoxic T killer cell granules?

Answer 39

The 2 chemicals in cytotoxic granules of killer T cells are perforin and granzyme.

Perforin punches holes in the membrane of the target cell. These allow granzymes to enter infected/ cancerous cells.

Granzymes are serine proteases that can cleave proteins once they are inside the cell. This causes apoptosis of target cells

Question 40

Describe the 3nd mechanism of CTLs once activated.

Answer 40

3rd way CTLs kill is the Fas-mediated apoptosis route.

This is induced when Fas molecules on the killer T cell bind to the Fas ligand on certain cells. This induces apoptosis of infected/cancerous cells

T cells become more sensitive to Fas-mediated apoptosis the longer they are activated. So this is a way to get rid of old/not needed T cells, preventing excessive response.

This also eliminates autoreactive T cells that recognise proteins in healthy cells