Lecture 8- Generation of antibody

Question 1

What secretes antibodies?

Answer 1

Important to note that B cells are not the cells which secrete antibody – these are PLASMA CELLS – B cells differentiate into plasma cells.

Question 2

How can the lymphocytes move into the lymph nodes?

Answer 2

Through the high endothelial venules via their interactions with the endothelium. And once the B cells are in the lymph nodes they can respond to specific chemokines that get them into specific locations

Question 3

Describe Plasma cell structure?

Answer 3

Plasma cells have unusual shaped nucleus, lots of heterochromatic regions, been described as a cartwheel nuclear structure – have a lot of RER and golgi apparatus. Since they’ve got to secrete a lot of protein.

Question 4

B cell development

Answer 4

B cells develop in the bone marrow
This is where they re-arrange their immunoglobulin genes –
This is independent of antigen but does depend on factors released by special cells in the bone marrow called stromal cells

Question 5

What happens after the B cells have re-arranged their immunoglobulin molecule in the bone marrow?

Answer 5

B cells then express their re-arranged immunoglobulin molecule on the membrane surface as IgM class
If any of these interact very strongly with self antigens in the bone marrow they are eliminated (called central tolerance)
These cells now mature (and additionally begin to express their re-arranged immunoglobulin molecule as an IgD class)

Question 6

If the B cell does not express a self antigen very strongly and survives the elimination round, what happens?

Answer 6

They mature and move out into the blood. They start expressing another class of immunoglobulins- the IgD class

Question 7

Describe the different stages in B cell development

Answer 7

1.B cell progenitor ->
2.Pro-B cell- starts to express surface markers e.g. CD19->
These B cells then rearrange their immunoglobulin and start to express the heavy chain. If they dont rearrnage their ig properly, they will die.
3.=Pre-B cell
4. Then start to express IgM= Immature B cell
5. Mature and move itno blood. Now expressing both IgM and IgD

Question 8

WHat happens when the B cell encounters their specific antigen in a lymph node?

Answer 8

they proliferate and eventually differentiate into plasma cells and long lived memory B cells

Question 9

Can the B cells that leave the bone marrow secrete antibody?

Answer 9

NO. the B cells that leave the bone marrow cannot secrete antibody. They have to differentiate into a different type of cell

Question 10

What do B cells require to be activated?

Answer 10

B cells usually require 2 signals to become activated (like T cells):
Signal 1 is recognition by the antigen specific membrane immunoglobulin (Ig) molecule
Signal 2 is most often from an interacting CD4+ T cell – this is T cell dependent stimulation

Question 11

What is T cell independent stimulation?

Answer 11

In some cases – some antigens e.g bacterial polysaccharides can deliver strong enough antigens to stimulate B cells without T cells – in this case the antibody is not usually as strong or efficient

Question 12

Where are B and T cells in the lymph node?

Answer 12

T cells are in the paracortex. B cells in the cortex

Question 13

How do B cells recognise their sepicific antigens in the lymph node? How do they antigens get to the B cells?

Answer 13

Antigen draining into a lymph node or spleen is collected by special macrophages (marginal sinus/marginal zone).

These macrophages allow conformational antigens to be recognised by any antigen specific B cell migrating through the cortex region of the lymph node

Question 14

Why do you need macrophages in the spleen and lymph nodes to activate the B cells?

Answer 14

You need more than one B cell receptor (Ig) on the surface of the cell needs to be stimulated – or cross-linked. Need more than one immunoglobulin to be activated on B cell in order to simulate activation

Question 15

What happens once the B cell is activated in the cortex? (sees its antigen on the macrophage)

Answer 15

It moves to the border between the cortex and the paracortex (because paracortex= where the T cells are). To get T cell help.

Question 16

What do T cells need to be activated?

Answer 16

Need dendritic cells.

So while the B cells are being activated by macrophages, the T cells are being activated by DCs in paracortex.

Question 17

What happens when the b cells and t cells have been activated and they start interacting?

Answer 17

They start to proliferate. The cells move to the medullary cords and start to form primary foci of proliferation. Then some cells migrate to the primary follicle

Question 18

What is the difference between primary foci and primary follicle?

Answer 18

primary foci= where cells begin to proliferate

primary follicle= area where B cells will move back to and where germinal centre will begin to form

Question 19

What does the B cell do when it interacts with its antigen on the macrophage?

Answer 19

The b cell internalises the antigen. It processes that antigen internally and presents peptides from it in MHC class II molecules. This can now be presented to T cells which is specific for that peptide.

Question 20

How do T cells interact with B cells?

3 ways

Answer 20

1. T cell recognises peptides presented on the B cell MHC class II.
2. Also interacts with B cell via co-stimulatory molecules. of particular note=CD40 and CD40Ligand
3. T cell produce cytokines which affect B cells proliferation and type of antibody produced

Question 21

Which process ensures that only the T cells that are specific for the antigen recognised by B cells make contact?

Answer 21

If a CD4+ T follicular help (Tfh) cell with the right specificity recognises the peptide antigen presented by the B cell it delivers the second signal to the B cell

Question 22

What is always the first type of antibody produced against an antigen?

Answer 22

IgM

Question 23

If doctors are testing whether you’ve recently been exposed to a pathogen what will they look for?

Answer 23

IgM because that’s the first type of antibody made before it undergoes class switching. (IgM’s sepcificty is not very good though)

Question 24

What does class switching allow?

Answer 24

Allows different types of antibodys to be made by plasma cells. e.g. IgG which will have different functions.

Question 25

What happens to the B cells after about 4 days?

Answer 25

some of the activated B cells and activated Tfh cells move to the cortex and enter regions of the lymph node called primary follicles Primary follicles contain a unique kind of antigen presenting cell called the Follicular Dendritic Cell - FDC

Question 26

What do follicular dendritic cells form?

Answer 26

FDCs form a network throughout the primary follicle

Question 27

What are follicular dendritic cells designed to do?

Answer 27

They are specially designed to hold antigen/antibody complexes on their surfaces in little nodules- iccosomes The antibody comes initially from the plasma cells in the extra-follicular region of the cortex

Question 28

What's special about follicular dendritic cells?

Answer 28

These dendritic cells aren't part of hamaetopoatic system. They're part of mesenchymal origin. And don’t express mhc class 2. very different from normal ones. These have specialised structures on their surface called iccosomes.

Question 29

What is an iccosome?

Answer 29

Antigen-antibody complex on surface of FDC

Question 30

What happens when B cell interacts with FDC?

Answer 30

B cell finds FDC that has an antigen it recognises it can bind to it, pull the iccosome off and internalise

Question 31

What are germinal centres?

Answer 31

Where the B cells are proliferating in the lymph node

Question 32

What are the 2 zones in germinal centre?

Answer 32

Dark zone and light zone Dark zone= where all proliferation happens. When B cells activated move into germinal centre they start to proliferate in dark zone. Called b cells centroblasts at this point. Then when stop proliferating and move into light zone. Now called they're called centrocytes,

Question 33

What happens when activated B cells enter the primary follicles?

Answer 33

As the activated B cells enter the primary follicles they down-regulate their Ig membrane receptors and proliferate extensively – they are now called centroblasts During this proliferation they undergo a process called affinity maturation This is the process where high affinity antibody is made

Question 34

Describe affinity maturation?

Answer 34

As the centroblasts divide they undergo hypermutation of their H and L chains of their particular Ig molecule – this will cause changes in the structure of the hypervariable regions of the antibody molecules =random process. So now have some cells make antibodies of higher or lower affinity than original. Only the higher affinity ones survive

Question 35

What happens to the surviving centrocytes after affinity maturation?

Answer 35

Meet up with activated Th cells again and differentiate into plasma cells or turn into memory B cells

Question 36

Describe class switching

Answer 36

Differentiation process. This is where the B cell is able to change its heavy chain constant region of the immunoglobulin gene from µ (IgM) to γ (IgG) or α (IgA) or ε (IgE) whilst keeping the same heavy chain variable and light chain (the antigen binding parts). Different antibodies bonds in different ways.

Question 37

What controls class switching?

Answer 37

Class switching is controlled by CD4 T helper cells and cytokines Without CD40/CD40L only make IgM-> hyperIgM syndrome. Different cytokines induce the production of different antibody classes e.g. IL-4 induces IgE, important against type 2 pathogens e.g. worms N.B. cytokines also influence how much antibody is made

Question 38

What are memory B cells?

Answer 38

Memory B cells can survive for long periods – they have undergone class switch and affinity maturation – so when they see antigen can respond with a very quick and efficient response

Question 39

What can combinations of cytokines do?

Answer 39

The combinations of cytokines can promote class switching too, not just single cytokines. (Can induce subtypes of immunoglobulins)

Question 40

What is the final stage of differentiation?

Answer 40

Final stage of differentiation is into memory cells and plasma cells

Question 41

Where does the plasma cell and memory b cell production happen?

Answer 41

Occurs in the “apical light zone” of the germinal centre

Question 42

What can turn a B cell into a memory cell?

Answer 42

If the B cell interacts with CD4+Tfh cell, the binding of CD154 on the Tfh cell to CD40 on the B cell turns it into a memory B cell – these can then leave the lymph node and go into the circulation

Question 43

How long do plasma cells survive?

Answer 43

Plasma cells can be short or long lived – some stay in lymph nodes or spleen and secrete antibody for a few weeks Others migrate to bone marrow and live for many months secreting antibody (the bone marrow is major source of antibody in the body, interesting)