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1. MICR 1220 > Antibodies > Flashcards

Antibodies Flashcards

1
Q

What is the structure of immunoglobulin?

A

2 heavy chains and 2 light chains linked by disulphide bonds
They can be cleaved into 2 parts using protease papain into a Fab (fragment antigen binding) fragment and an Fc region (fragment crystallisation)

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2
Q

What are the FAB and Fc regions of an immunoglobulin?

A

The Fab fragment region is responsible for antigen binding (which there are 2 antigen binding domains per molecule)
The Fc region is bound by Fc receptors expressed by phagocytes and other immune cells

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3
Q

What are the two types of light chain?

A

λ (lambda)

k (kappa)

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4
Q

What are the two types of domain?

A

Variable domain - made up of 2 domains: Variable light (VL) and Variable Heavy (VH)
They vary which defines antigen specificity

Constant domain - made up of 4 domains: Constant Light (CL) and 3 Contant Heavy (CH1, CH2 and CH3) 
Within the same class they don't differ
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5
Q

What is the antigen binding site made up of?

A

The variable domain - Variable light (VL) and Varibale Heavy (VH)

There are loops that project form theses regions - called complementary determining regions (CDR) and hypervariable loops
The loops help determine specificty/complementarity

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6
Q

How were epitopes discovered?

A

Hapten - a small molcule that can act as an epitope but wouldn’t induce antibodies on their own

  1. Hapten is combined with a protein (ovalbumin)
  2. Give the compound to a mouse
  3. This will produce antibodies to ovalbumin and the hapten
  4. Take the hapten antibodies and bind to hapten in vitro
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7
Q

What can act as an epitope?

A

Any chemical can act as an epitope (hapten) in this way; inorganic chemical, peptide, sugar, lipid, nucleic acid

Antibodies recognise antigenic epitopes by molecular complementation i.e. can’t have steric interferences/hindrances

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8
Q

What is important about the epitope shape?

A

Variation in hapten structure (chemical positions of functional groups) can lead to different affinities of binding to an antibody

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9
Q

What binding do antibodies use?

A 
Antibodies use non-covalent bonds to bind to antigenic epitopes:
Electrostatic interactions
Hydrogen bonds
Hydrophobic interactions
Van-der Waals forces

A complementary binding surface on an antibody maximises the non-covalent interactions with the epitope

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10
Q

What are the types of epitopes?

A

Continuous epitopes - residues that make up the epitope are all directly linked

Discontinuous epitopes - formed by the apposition of distant residues as a result of molecular folding
Discontinuous epitopes are typically lost by unfolding or denaturing protein antigens

Epitopes can be 12 - 16 amino acids or 5 - 6 sugar residues

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11
Q

What are the 5 classes of immunoglobulins? What are they distinguished by?

A

IgM, IgG, IgA, IgE and IgD

They are distinguished by their heavy chains:
IgM - µ (mu) heavy chain 
IgG - γ (gamma) heavy chain
IgA - a (alpha) heavy chain
IgE - ε (epsilon) heavy chain 
IgD - δ (delta) heavy chain
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12
Q

Describe the structure of IgM?

A

Looks like a star

µ heavy chain has 4 CH domains
It is pentameric (970kDa): 10 antigen binding sites
The J chain promotes polymerisation
The monomers are linked by disulphide bonds to each other and to the J chain
Can be imaged by electron microscopy

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13
Q

What are the functions of IgM?

A

It is the first antibody secreted in response to a foreign antigen (primary response)

Neutralises antigens
Agglutinates (clumps) microbes
Activates classical complement pathway
IgM is effective in limiting the spread of microorganisms via the bloodstream

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14
Q

Facts about IgM?

A

Confined to the vascular system due to large size

It has a low affinity for an antigen, but has 10 binding sites so can vind to polyvalent antigens e.g. Bacterial surfaces

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15
Q

Describe the structure of IgG?

A

Looks normal - monomeric

4 distinct isotypes: IgG1, IgG2, IgG3 and IgG4
IgG1, IgG2 and IgG4 - 146 kDa
IgG3 - 165 kDa
Each gamma-heavy chain has 3 CH domains

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16
Q

What are the functions of IgG?

A

Neutralises antigens
Bound by Fc receptors on phagocytes
Activates the classical complement pathway
During pregnancy it is actively transported across placenta into the foetus to protect new born babies for the first 3-6 months

17
Q

Facts about IgG?

A

Produced in secondary response to an antigen by B-cells in the lymph nodes and spleen
Major antibody in normal human blood, IgG1 is the most abundant isotype in the blood (9 mg/ml in serum)
Can enter the extra-vascular sites due to small size i.e. the tissues of the body

IgG opsonises microbes for uptake by phagocytes

18
Q

Describe the structure of IgA?

A

Either normal - monomeric (160 kDa) or two joined together at the bottom - dimer (320 kDa)
In the dimer disulphide bonds and the J chain hold them together

19
Q

What are the functions of IgA?

A

The principal function of IgA is to neutralise antigens:
IgA prevents bacterial toxins binding their cellular targets
IgA inhibits microbial adhesion to epithelia
IgA inhibits viral infectivity

20
Q

Facts about IgA?

A

IgA is produced by B-cells in the mucosal associated lymphoid tissues during secondary immune response
Majority is secreted onto mucosal epithelial surfaces as Ig receptor transports it across the epithelium

21
Q

Describe the structure of IgE?

A

Monomeric - 188 kDa

The ε heavy chain has 4CH domains

22
Q

What are the functions of IgE?

A

Important for immune responses against helminths (worms) and other parasites
Also involved in type I hypersensitivity responses (allergic reactions) eg hay fever, food allergies
Binding of IgE molecules to polyvalent antigens cross-links Fc(epsilon) receptors and triggers release of inflammatory mediators

23
Q

Facts about IgE?

A

Produced in response to some antigens, usually by B-cells in the mucosal associated lymphoid tissues• Low concentration in the blood
Vast majority of IgE is bound to Fc(epsilon) receptors on mast cells, basophils and eosinophils

24
Q

Describe IgD?

A

Monomeric - 160 kDa
The δ heavy chain has 3CH domains

Acts as antigen receptor on immature B-cells and is present in the blood at very low concentrations
No other biological function has yet been described

25
Q

What is antibody diversity? How does it arise?

A

Antibody diversity refers to the array of different antigenic specificities produced in the humoral immune response
Antibody diversity is generated by recombination of the antibody genes during the antigen-independent differentiation of B-cells in the bone marrow (antibody gene rearrangement)

26
Q

Facts about antibody diversity?

A

5 X 10^13 different specificities of antibodies can be produced
This allows enough antibodies for every antigen it encounters

The total number of genes in the human genome is insufficient to accommodate one complete gene for each antibody molecule (there are only around 21,000 protein coding genes in the human genome)
Three of these genes are for immunoglobulin

Solution: Diversity is achieved by recombination of less than 1,000 gene segments in the DNA of developing B-cells

27
Q

How does specificity arise?

A

Antigenic specificity of an antibody arises from the sequence of the VL and VH domains

In order to encode an antibody molecule, immunoglobulin genes must combine

28
Q

What are all the gene segments that are used within the chains?

A

V coded for by variable (V)-gene segment [98aa]
D coded for by diversity (D)-gene segment [16-21aa]
J coded for by joining (J)-gene segment [1-15aa]
C coded for by constant gene segment

29
Q

What is the Variable light chain coded for by? Example?

A

Variable (V) and Joining (J)

K-light chain of chromosome 2
39 V gene segments and 5 J gene segments
It also has a constant gene segment (C)

30
Q

What is the Variable heavy chain coded for by? Example?

A

Variable (V), Diversity (D) and Joining (J)

Chromosome 14 Heavy chain
46 V gene segments, 23 D gene segments and 6 J gene segments
It has 5 constant gene segments

31
Q

What is an overview of the gene recombination?

A

Immunoglobulin genes are non-functional until they are rearranged
They are rearranged during antigen independent differentiation of B cells in the bone marrow

Each light and heavy chain uses only one of each gene segment
This is called V(D)J recombination
It allows a large number of possible combinations from only a few gene segments

32
Q

How is the variable light chain developed?

A

Somatic recombination VJ joining
Any combination is technically possible during recombination
After this recombination we get transcription to form a primary RNA transcript
Splicing will then remove J segments that aren’t needed to form mRNA in a virgin B-cell
The light chain mRNA is then translated into a protein in the RER

33
Q

How is the variable heavy chain developed?

A

First there is somatic recombination DJ joining
Then there is somatic recombination VDJ joining to form a rearranged DNA pre-B-cell
Transcription will then form a primary RNA transcript
RNA splicing will remove unwanted J segments to form mRNA in virgin B-cells
This mRNA is translated into the heavy chain proteins in the RER

34
Q

What happens when the VL and the VH chains are produced?

A

The heavy chains can now associate with light chains in the ER to form antibody molecules
They are sent to the Golgi which can then be secreted

35
Q

What is combinatorial diversity?

A

This refers to the different combinations of heavy and light chains that can be used to form an antibody
Theoretically any heavy and light chain can interact to form an antibody molecule
Combinatorial diversity further increases antigen specificity

36
Q

What is junctional diversity

A

Junctional diversity describes the DNA sequence variations introduced by the imprecise joining of gene segments during the process of V(D)J recombination
The random generation of sequences and the addition of nucleotides being added and removed at random (at the sites of V, D and J gene segments)

The junction corresponds to the 3rd hypervariable loop of the variable domain

37
Q

How can immunoglobulins switch class?

A

B-cells switch class as the immune response matures
Naïve B-cells exit the bone marrow expressing IgM and IgD on their cell surface
IgM is the first immunoglobulin secreted in response to antigens
As the humoral response matures B-cells undergo class switching and one IgG, IgA and IgE are expressed instead of IgM/IgD by B-cells
Class switching tailors the humoral response to the antigen

38
Q

What is significant with immunoglobulin switching?

A 
Only the constant regions of the heavy chains are switched
Therefore class switching doesn't alter the antigenic specificity because the light chain and VH domains are maintained

1. MICR 1220 (7 decks)

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