1.06 - B-Cells & Antibodies

Question 1

What are antibodies?

Answer 1

Proteins secreted by activated B cell
The same protein is located on the B-cell surface but is then known at the B-Cell receptor
Each B-Cell secretes a unique protein

Question 2

What can antibodies bind to?

Answer 2

A variety of substances
Proteins
Lipids
Polysaccharides

Question 3

What is the affinity of antibodies?

Answer 3

It is the binding strength between the antibody and antigen at a single binding site

Question 4

What are the roles of antibodies in the immune system?

Answer 4

Neutralisation: antibodies prevent bacterial adherence and also block toxins from working
Opsonisation: surrounding a cell or microorganism and marking it for phagocytosis
Complement activation: Antibodies bound to target will activate complement cascade

Question 5

Describe the structure of antibodies

Answer 5

Consist of four chains: 2 heavy (joined by sulphide bonds) and 2 light chains.
Each chain contains a variable and constant region
The constant region determines the antibody isotope
The variable region determines antibody specificity
The variable region contains three areas of increased variability known as complementarity determining regions (CDRs)

Question 6

What are the five antibody isotypes?

Answer 6

IgG
IgM
IgA
IgE
IgD

Question 7

What are the effector functions of IgG?

Answer 7

Opsonisation of antigens for phagocytosis by macrophages and neutrophils
Activation of classical pathway of complement
Antibody-dependent cell-mediated cytotoxicity mediated by natural killer cells
Neonatal immunity: transfer of maternal antibody across the placenta and gut
Feedback inhibition of B cell activation

Question 8

What are the effector functions of IgM?

Answer 8

Activation of the classical pathway of complement

Antigen receptor of naive B lymphocytes

Question 9

What are the effector functions of IgA?

Answer 9

Mucosal Immunity: secretion of IgA into the lumen of the GIT and respiratory tracts
Activation of complement by the lectin or alternative pathway

Question 10

What are the effector functions of IgE?

Answer 10

Mast cell degranulation (immediate hypersensitivity reactions

Question 11

What are the effector functions of IgD?

Answer 11

Antigen receptor of naive B lymphocytes

Question 12

What are the structures of the 5 antibody isotypes?

I.e. Monomer, Dimer etc.

Answer 12

IgG: Monomer
IgM: Pentamer
IgA: Dimer
IgE: Monomer
IgD: Monomer

Question 13

Which antibody type is not secreted?

Answer 13

IgD

Question 14

What are the two separate steps in the generation of antibody diversity?

Answer 14

VDJ recombination: different genes are randomly selected during B cell maturation
B cell receptor is mutated in germinal centre during an immune response to increase its affinity for the antigen

Question 15

Describe the process of B-cell receptor light chain rearrangement and production

Answer 15

Germline DNA contains 4 segments: L-V—J–C
Somatic recombination joins the V & J segment in the rearranged DNA: L-VJ–C
Transcription then occurs to produce primary transcript RNA: L-VJ–C
Splicing of the RNA removes the introns and brings the L and VJ segments together as well as the VJ next to the C: LVJC
Translation of the mRNA produces the functional polypeptide chain that is sent to the cell surface

Question 16

Describe the process of B-cell receptor heavy chain rearrangement and production

Answer 16

Germline DNA contains 5 segments: L-V—D—J–C
The C region contains 4 exons (3 for constant region. 1 for hinge region)
Somatic recombination brings D-J segments together in rearranged DNA: L-V—DJ-C
Another stage of somatic recombination brings the V to the DJ segment: L-VDJ—C
Transcription forms primary transcript RNA: L-VDJ–C
Splicing of the introns forms mRNA: LVDJC
Translation forms the functional polypeptide chain that is sent to the cell surface

Question 17

What are the names given to the cell in each stage of B-Cell development?

Answer 17

Stem Cell
Early Pro-B cell
Late Pro-B-cell
Large pre-B-cell
Small pre-B-cell
Immature B cell
Mature B cell

Question 18

Describe the Stem Cell in B cell development, with relation to H-Chain genes, L-chain genes and any surface Ig present

Answer 18

H-Chain Genes: Germline
L-Chain Genes: Germline
Surface Ig: Absent

Question 19

Describe the Early Pro-B-Cell in B cell development, with relation to H-Chain genes, L-chain genes and any surface Ig present

Answer 19

H-Chain Genes: D-J Rearranging
L-Chain Genes: Germline
Surface Ig: Absent

Question 20

Describe the Late Pro-B-Cell in B cell development, with relation to H-Chain genes, L-chain genes and any surface Ig present

Answer 20

H-Chain Genes: V-DJ Rearranging
L-Chain Genes: Germline
Surface Ig: Absent

Question 21

Describe the Large Pre-B-Cell in B cell development, with relation to H-Chain genes, L-chain genes and any surface Ig present

Answer 21

H-Chain Genes: VDJ Rearranged
L-Chain Genes: Germline
Surface Ig: u Chain transiently at surface as part of pre-B-cell receptor. Mainly intracellularly

Question 22

Describe the Small Pre-B-Cell in B cell development, with relation to H-Chain genes, L-chain genes and any surface Ig present

Answer 22

H-Chain Genes: VDJ Rearranged
L-Chain Genes: V-J Rearranging
Surface Ig: Intracellular u chain

Question 23

Describe the Immature B-Cell in B cell development, with relation to H-Chain genes, L-chain genes and any surface Ig present

Answer 23

H-Chain Genes: VDJ Rearranged
L-Chain Genes: V-J Rearranged
Surface Ig: IgM expressed on cell surface

Question 24

Describe the Mature B-Cell in B cell development, with relation to H-Chain genes, L-chain genes and any surface Ig present

Answer 24

H-Chain Genes: VDJ Rearranged
L-Chain Genes: V-J Rearranged
Surface Ig: IgD and IgM made from alternatively spliced H-chain transcripts expressed on cell surface

Question 25

Where are the two main phases of B-Cell development located?

Answer 25

Bone marrow and Periphery

Question 26

Where are B-cells located?

Answer 26

Developing B-cell and long-lived plasma cells are found in the bone marrow Mature B-cells are found in the spleen and lymph nodes and circulate in the blood

Question 27

Describe T-Dependent Antibody Synthesis

Answer 27

Antibody synthesis in response to protein antigens Leads to formation of germinal centre Leads to formation of memory B cells and long-lived plasma cells Gives rise to secondary humoral response

Question 28

What are the stages of T-Dependent antibody synthesis?

Answer 28

``` Activation and migration of T and B lymphocytes T:B cell interaction B-cell differentiation: Ig secretion, isotype class switching Germinal centre reaction: affinity maturation, long lived plasma cells, isotype switching, memory B cells ```

Question 29

What are the two things B cells can do after activation at the T:B cell border?

Answer 29

Move to the extra follicular area and become plasma blasts and plasma cells and secrete antibody Move to the follicle and proliferate becoming a germinal centre to make a higher quality antibody

Question 30

What interaction is important for class switching of antibodies?

Answer 30

CD40 on B-cells and CD40L on helper T cells

Question 31

Describe the germinal centre

Answer 31

B-cells in the follicle rapidly proliferate generating the germinal centre Centroblasts are rapidly proliferating germinal centre B cells in which the B cell receptor is being mutated Centrocytes are germinal centre B cells that reexpress their new B cell receptor to check whether it is of higher affinity Centrocytes with Bcell receptors of higher affinity get survival signals from follicular dendritic cells and T cells

Question 32

Describe somatic hypermutation

Answer 32

Process by which germinal centre B cells mutate their B cell receptor to increase its affinity Random mutations (some advantageous, some deleterious) are made by special enzymes expressed in these cells Accumulation of advantageous mutations is known as affinity maturation B cells with B cell receptors with higher affinity for the antigen survive and become memory B cells and long- lived plasma cells

Question 33

What are AID and UNG and what is their function?

Answer 33

``` They are enzymes important for somatic hypermutation and class switching recombination Mutate and remove certain residues to cause mutation ```

Question 34

Describe class switch recombination

Answer 34

``` Also known as isotype switching Make an antibody type that is appropriate for a particular function Class switch to IgA, IgG or IgE While germinal centre formation is not absolutely required for class switch recombination, most affinity matured B cells will make IgG antibody ```

Question 35

What are the two phases of B cell maturation?

Answer 35

Antigen independent B cell maturation | Antigen dependent B cell maturation

Question 36

Describe Antigen independent B cell maturation

Answer 36

Occurs in bone marrow and involves production of naïve B cell from stem cell progenitor Genes rearranged to make functional B cell receptor Receptor editing occurs if B cell receptor is not functional B cells that bind strongly to antigen are removed

Question 37

Describe Antigen dependent B cell maturation

Answer 37

Occurs after naïve B cell encounters an antigen that binds its receptor Rearranged VDJ segments mutated to make higher affinity antibodies B cells that bind strongly to antigen are given survival signals

Question 38

Describe Plasma Cells

Answer 38

Antibody producing cells No longer have cell surface B-Cell receptor Increased cytoplasm and rough ER

Question 39

Describe T independent B cell responses

Answer 39

B cells can be activated by cross-linking of multiple B cell receptors, in the absence of T cells This requires multivalent antigen - usually bacterial polysaccharides, lipopolysaccharides or multimeric proteins T independent reactions do not result in germinal centre reactions, somatic hypermutation, memory B cells or long-lived plasma cells