Adaptive Immunity B cell

Question 1

What is the definition of an antibody?

Answer 1

A soluble protein which is secreted by B lymphocytes that bind to and mediate the destruction of invading micro-organisms.

Question 2

Where do B and T cells develop from?

Answer 2

common lymphoid progenitor cell

Question 3

Where do B cells migrate to after maturing in bone marrow?

Answer 3

lymph nodes and spleen

Question 4

What turns a B cell away from being naive?

Answer 4

encountering infectious microorgansims

Question 5

How will they respond to infectious micro-organsisms?

Answer 5

T cell help to proliferate and secrete a soluble form of antibodies

Question 6

How do antibodies work? What do they inhibit and induce?

Answer 6

They bind to infectious micro-organisms and prevent infection.
They inhibit attachment to host tissue, activate complement, enhance phagocytosis, induce degranulation of mast cells .
Each B lymphocyte produces antibodies of unique specificity.

Question 7

What is the general antibody structure?

Answer 7

• 2 identical heavy chains of 50-6- thousand molecular mass
• heavy chains are linked by disulphide bonds to each other
• heavy chains linked by disulphide bonds to one of the two identical light chains
• light chains have a mass of 25 thousand
• Fab binds antigen
• Fc is needed for effective antibody function
• N terminal part of the heavy chain forms two protein domains which are linked by the hinge region to C terminal domains
• N terminal protein domains wrap around corresponding C terminal domains of the other heavy chain
• the two N terminal domains fold together with the light chains and form identical binding sites

Question 8

Do protein domains have the same or different structure?

Answer 8

All have a similar structure

Question 9

What are the two variable domains called?

What do they form?

Answer 9

VH and VL

They form the antigen binding site.

Question 10

What do the constant regions vary between?

Answer 10

Different classes of antibody

Question 11

What does flexibility in the hinge region increase?

Answer 11

Ability for the antibody to bind to two sites on the antigen to increase binding strength.

Question 12

What does the C terminal part of the heavy chain mediate?

Answer 12

Effector functions including complement activation, binding to Fc receptors on immune cells, transfer of antibodies to specific anatomical sites.

Question 13

What is an epitope?

Answer 13

A specific site on the antigen that binds to the antibody.
Large antigens will have more than 1.
Antigens may have severe epitopes that different antibodies of different specificity can bind.

Question 14

Explain the structure of the antigen binding site

Answer 14

Formed of 3 loops from each of the variable heavy and light chain domains between beta strands.
These loops are called complementarity determining regions. Loops from VH and VL domains contribute to antigen binding.
CDR1 as the most N terminal, then CDR2 and CDR3 as closest to the C terminal part of the domain.
CDR3 is often the largest loop and contributes more than the others to the affinity and specificity of antigen binding.

Question 15

What are the 3 effector functions mediated by the Fc region of the antibody?

Answer 15

1. Activation of complement - c1q activated by antibodies in immune complexes or bound to surface of pathogen. Lysis or phagocytosis via complement receptors.
   (binding to antigen to form an immune complex causes a conformational change in antibody allowing the Fc domain to bind to C1q of the classical pathway leading to cell lysis or inducing phagocytosis of the immune complex via complement receptors)
2. Immune complexes can bind directly to Fc receptors and be phagocytosed. If the antibody is bound to a large micro-organism, binding of the antigen to Fc receptor may cause degranulation.
3. Transport across epithelium to mucosal surfaces
4. Transfer of maternal antibodies through placenta to protect foetus

Question 16

Antibodies:

1. How many classes are there?
2. How do they differ?
3. Explain the structure of the IgM antibody
4. What is the J chain essential for?
5. Explain the structure of the IgD antibody
6. Give small detail on class switching
7. When is IgG produced?
8. Give IgA structure
9. Give IgE structure

Answer 16

1. 5
2. Number of immunoglobulin domains inn Fc regions and whether they are momeric or more complex
3. Comprises of 5 identical antibodies each with an additional immunoglobulin domain in the Fc portion. antibodies are linked by disulfide bonds between CH3 and CH4 domains. These disulfide bonds are always the same length. The ring is complete by a protein called the J chain which is linked by disulphide bonds to each of the antibodies at the end of the Pentomer.
4. The J chain is essential for oligermisation, stabilising and auto conformation of the CH4 domain of each individual IgM that allows close packing of this part of the antibody to form the pentomer.
5. IgD is a monomer produced a low levels. All naive B cells produce IgD and IgM which both have an identical binding site for pathogens.
6. As the antibody response continues, B cells change the class of antibody to produce antibodies more suitable for elimination of the pathogen. Class switching involves DNA recombination in the region of the chromosome coding for the antibody heavy chain. As a result, the class switch antibody maintains the original VL and VH domains and therefore maintains antigen specificity.
7. IgG is the main antibody in serum and can be transferred to the foetus.
8. IgA is dimeric and dimerisation is facilitated and stabilised by the J chain protein which is similar to what the J chain does to IgM. IgA is produced in large quantities but most of it is actively transported across mucosal epithelium and present on external surfaces to prevent pathogen entry.
9. IgE has a heavy chain comprising of 4 constant domains is produced in low quantities and almost all of it is associated with high affinity Fc receptors on the surface of mast cells and eosinophils.

Question 17

How many subclasses does IgG have?
What antibody is a strong activator of complement?
Which antibodies immune complexes bind strongly to Fc receptor?
What does IgA usually promote?
What does IgE bind strongly to?

Answer 17

4

IgM followed by IgG1 and IgG3

IgG1 and IgG3

Phagocytosis by binding to Fc alpha receptors on macrophages and neutrophils

Fc epsilon receptors on mast cells, eosinophils and basophils

Question 18

Explain briefly the antibody mediated activation of complement and phagocytosis

Answer 18

Antibody meditated activation of complement can kill pathogens by the complement mediated lysis by the last component, c9, forming pores in the pathogen surface.

Alternatively, complement receptor mediated phagocytosis of C3b modifying pathogen, can lead to antigen destruction.

Question 19

What antibodies induce phagocytosis?

Answer 19

Ig1 and IG3 antibodies induce a TH1 response and form immune complexes that bind with high affinity to Fc gamma receptors on phagocytic cells leading to destruction of the pathogen.

Question 20

How do we get Th2 responses to the pathogens?

Answer 20

Cross-linking pathogen specific IgE antibodies that are bound with high affinity to the Fc epsilon receptors on the surface of mast cells or eosinophils.
IgE are largely bound to high affinity Fc epsilon receptors resulting in degranulation (release of histamine, serotonin and proteases). This attracts lymphocytes, eosinophils, neutrophils and macrophages.

Question 21

What is the immunoglobulin heavy chain gene locus similar in structure to and why?

What codes for the constant region of the heavy chain?

Answer 21

T cell receptor beta chain locus as it has several V, D and J segments that can be combined by DNA rearrangement to produce a functional exon that codes for the heavy chain variable domain.

Exons

Question 22

How many gene loci code for light chains?

What do these include and what is this similar ti?

Answer 22

2

Similarly to the T cell receptor alpha chain, these include V and J segments that are recombined to form the functional exons that code for the variable light chain domains.

Question 23

What do all antibodies express?

What happens during maturation in bone marrow?

What can the light chain do if it has bound to self antigen?

Answer 23

IgM monomer and IgD with the same antigen binding site.
Each B cell expresses antigen receptors of unique specificity.

Negative selection of B cells. Cells which recognise self antigen either undergo apoptosis of may undergo further re-arrangement of the light chain for a limited period. 
This process (receptor editing), underlines the importance of maintaining diversity in B cell antigen recognition. 

Can change antigen binding site to one that does not recognise self antigens.

Question 24

What is B cell requirement less strict than for T cells?

Answer 24

B cells are not tissue damaging (unlike cytotoxic T cells).

B cells need T cells to function.

Question 25

What is odd about Igd?

Answer 25

IgD function is not clear. It has been maintained in evolution suggesting its important. Reports that is is more abundant in respiratory mucosa so may be needed for early response to infection. It is odd as it is the first antibody produced in antibody response along with IgM.

Question 26

Explain how activation of B lymphocytes in spleen or lymph nodes occurs (3 things)

Answer 26

Here they may encounter antigen in soluble form or immune complexes expressed on the surface of macrophages. 1. B cell receptor binds to antigen. Antigen binding induces the first signalling molecule via the B cell receptor complex. B cells act as antigen presenting cells for T cells. The B cell receptor is internalised together with the bound antigen which then undergoes proteolytic antigen processing and the resulting antigenic peptide in association with MHC 2 proteins are exported to the cell surface 2. Antigen specific binding of a helper T cell through the T cell receptor complex provides the second activating signal to the B cell via CD154 which binds to receptor CD40 on the B cell 3. T helper cell secretes cytokines that stimulates B cell proliferation and differentiation

Question 27

Why do B and T cells recognise the same antigen?

Answer 27

The requirement for antigen specific binding T helper cell recognising the same antigen as the B cell ensures that the B and T cells are directed against the same pathogen and co-operate to form an effective immune response.

Question 28

What happens to the B cell after activation?

Answer 28

Rapid proliferation and differentiation of plasma cells which migrate to bone marrow and produce large antibody quantities (IgM and IgD). ``` The proliferating B cell gives rise to plasma cells which migrate to bone marrow and churn out large quantities of pentameric IgM. IgD is produced at a lower rate. Plasma cells are short lived. Some of the cell may differentiate into memory B cells but at this stage, most of the cells undergo class switching (resulting in antibody of different classes being produced while maintaining specificity for antigen binding). ```

Question 29

What does class switching in B cells require? Is this reversible?

Answer 29

``` Class switching involves further DNA recombination of the heavy chain locus with exons coding for particular heavy chain classes, together with religation. The gene encoding whatever class of heavy chain is to be expressed by the particular B cell is closest to the functional V region. ``` Class switching is irreversible as DNA has been intervened but further switching of genes that are downstreamed to the changed gene is possible

Question 30

``` What is class switching controlled by? What is the point of it? ``` Explain the differences in the antibody action

Answer 30

Th cell cytokines. The point of class switching is to provide a more specific antigen response either to the type of pathogen or to the location of the pathogen (e.g, transport of IgA to external mucosal surfaces). Th1 response to intracellular pathogens - IgG1 and IgG3 via complement and Fc receptor binding Th2 response to multicellular parasites - IgE for degranulation of mast cells and eosinophils IgA produces at high levels and transported to mucosal surfaces to block pathogen entry

Question 31

What happens at the same time as class switching? What is it induced by? What is it? What does it result in?

Answer 31

At the same time that B cells are undergoing class switching in lymphoid tissues, somatic hypermutation also occurs. This is induced by the same enzyme that induces immunoglobulin class switching in both cases by un-stabilising normal GC pairing. Repairing this introduces DNA mutations. It is a mechanism that introduced point mutation into variable region exons during RNA transcription. Some mutations will result in loss of binding to the original antigen but some result in higher affinity binding. Competition for binding to antigen results in selection of B cells that express receptors and antibodies with higher affinity to the antigen (affinity maturation).

Question 32

Give some details on the time course of B cell response in serum?

Answer 32

Takes over a week for the antibodies to appear and these are normally IgM. Class switching produces IgG and IgG memory cells are also produced. ``` Memory IgG cells persist after immune response dies resulting in a stronger and faster second response to the same pathogen where IgG antibodies predominate. The memory cells represent a higher pool of antigen specific B cells and there will be less delay to class switching for IgG. ``` IgM response which are naive cells encountering antigens for the first time. IgM response may be reduced when the quantity of pathogen is lowered due to the greater and immediate IgG response.