Lecture 15: Immunoglobulin Structure and Function

Question 1

What happens when a B cell recognises the epitope of an antigen?

Answer 1

B cells undergo clonal selection, proliferation and generation of plasma cells which secrete antibody + memory cells which persist to combat next infection

Question 2

How many different encoded specificities of B cells are there in humans?

Answer 2

~10^7 different encoded specificities

Question 3

What does a given antigen bind to on its specific B cell?

Answer 3

surface Ig

Question 4

What is surface Ig?

Answer 4

immunoglobulins which are embedded in the B cell membrane

Question 5

What is the role of surface Ig?

Answer 5

acts as the B cell antigen receptor and can transduce activation signals upon antigen binding

Question 6

How many copies of surface Ig does each B cell express?

Answer 6

multiple copies ~10^5 per cell that bind the same antigen

Question 7

What is secreted Ig secreted by?

Answer 7

activated B cells (plasma cells)

Question 8

What is the difference between secreted IgG and membrane IgG?

Answer 8

secreted IgG has all the same regions as membrane IgG except it has a tail piece and membrane IgG has a cytoplasmic tail + a hydrophobic transmembrane region

Question 9

What is the structure of immunoglobulins? How are they held together?

Answer 9

consists of two heavy and two light chains

chains are held together by disulphide bonds

Question 10

What is the structure of the Ig domains?

Answer 10

~110 amino acids long

2 layers of beta-pleated sheet, 3-5 antiparallel strands

Question 11

What do inward pointing hydrophobic residues form?

Answer 11

a stable core

Question 12

What are light chains consisted of?

Answer 12

two Ig domains

Question 13

What are heavy chains consisted of?

Answer 13

four to five Ig domains (depending on the class of antibody)

Question 14

What regions do chains have? What is the function of these regions?

Answer 14

variable (V) and constant (C) regions
variable - antigen binding
constant - effector function

Question 15

What is the role of the hinge which links antigen-binding arms?

Answer 15

allows flexibility in binding to multiple antigens

Question 16

What are the two functionally distinct parts of Ig?

Answer 16

the Fc part and the Fab part

Question 17

What is the Fc part of Ig?

Answer 17

tail constant region, can be 1 of 5 isotypes

Question 18

What is the Fab part of Ig?

Answer 18

antigen binding, hypervariable (humans can make ~10^12 antibody specificities)

Question 19

What can the immunoglobulin molecule be readily cleaved into using specific enzymes?

Answer 19

functionally distinct fragments

Question 20

How does pepsin cleave Ig?

Answer 20

separates both Fab regions from digested Fc region

Question 21

How does papain cleave Ig?

Answer 21

releases individual Fab and Fc fragments

Question 22

What is most of the variability between different Igs within? What do these form?

Answer 22

three regions of Vh and Vl (hypervariable regions)

these form the antigen binding site

Question 23

How long are hypervariable regions?

Answer 23

~10 amino acids in length

Question 24

What are hypervariable regions also known as?

Answer 24

CDRs or complementarity determining regions

Question 25

In which CDR region is there the most variability?

Answer 25

in the CDR3 region

Question 26

How can immunoglobulins differ in their light chain?

Answer 26

either have a kappa or a lambda light chain | the same type of light chain is found on both arms of a single Ig molecule

Question 27

How can immunoglobulins differ in the constant region of their heavy chain?

Answer 27

``` different class or isotype e.g. mammals have 5 isotypes: IgA, IgD, IgG, IgE and IgM (designated by Greek letter) ```

Question 28

What are the properties of IgG?

Answer 28

high affinity monomer (bivalent) major class in secondary response large hinge region (i.e. flexible arms)

Question 29

What are the properties of IgM?

Answer 29

``` has extra constant Ig domain pentamer (10 valency) Ig oligomerization via J-chain first Ig produced high avidity although low affinity ```

Question 30

What is affinity?

Answer 30

the strength of binding of one molecule to another at a single site

Question 31

What is avidity?

Answer 31

sum total of the strength of binding between two molecules or cells to one another (where multiple binding sites are involved)

Question 32

Do multimeric Ig have high or low avidity? | Do monomeric Ig have high or low avidity?

Answer 32

multimeric Ig have high avidity | monomeric Ig have low avidity

Question 33

What are the properties of IgA?

Answer 33

dimer (tetravalent) major Ig secreted into GI and respiratory tracts 26aa tailpiece links to J-chain

Question 34

What are the properties of IgD?

Answer 34

low abundance and mainly cell-associated

Question 35

What are the properties of IgE?

Answer 35

involved in response to parasites and allergic reactions binds to mast cells, induces degranulation extra C domain

Question 36

Where are the different Ig isotypes distributed?

Answer 36

``` IgG = serum and lymph IgM = serum IgA = serum and secretions (e.g. tears, GI tract) IgD = serum? IgE = beneath epithelial surfaces ```

Question 37

What does the constant region confer?

Answer 37

functional specialisation on the antibody

Question 38

What are different classes of Igs distinguished by?

Answer 38

the structure of their heavy-chain constant regions

Question 39

What are antibody effector functions?

Answer 39

neutralisation, opsonization and complement activation

Question 40

What is the role of the Fab region?

Answer 40

involves antigen recognition | direct mechanism

Question 41

What is the role of the Fc region?

Answer 41

indirect / targeting mechanisms

Question 42

What is neutralisation (direct mechanism)? -> IgG, IgA, IgM

Answer 42

Ab binding to proteins used by viruses or bacteria which are essential for attachment can block infection

Question 43

What is opsonization (indirect mechanism)? -> IgG1 > IgG3 > IgG4 = IgA

Answer 43

coating of pathogen with specific Ab -> binding to phagocytes via FcR -> enhanced phagocytosis of bound particle

Question 44

What is antibody-dependent cell-mediated cytotoxicity (indirect mechanism)? -> IgG1 = IgG3

Answer 44

Ab binds to target cell via variable region e.g. virus infection cell Ab binds to NK cell via FcyRIII receptor (CD16) cross-linking of FcR triggers NK cell killing

Question 45

What is degranulation and what is it carried out by (indirect mechanism)? -> IgE

Answer 45

IgE binds to parasite or allergen and FceR on immune cell cross-linking of FceR triggers degranulation -> release of inflammatory mediates such as histamine carried out by eosinophils, basophils and mast cells

Question 46

What is complement activation (indirect mechanism)? -> IgM > IgG1 = IgG3 > IgG2 = IgA

Answer 46

Ab binding to component C1q via Fc initiates the classical complement cascade only IgG and IgM complexes can bind C1q and free Ab does not activate C1q