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Microbiology and Immunology - SGUL (Sem 4) > Humoral Immunity > Flashcards

Humoral Immunity Flashcards

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1
Q

Revision: Describe the structure of an antibody (Immunoglobulin)

A
  • Variable region - different
  • Constant region - Same for all Ab of the same class
  • Antigen binding site
  • Light chain - k or lambda
  • Heavy chain - 9 different types
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2
Q

Describe the 2 different types of immunoglobulins

A
  • Membrane bound receptor
  • secreted antibody
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3
Q

Describe the structure of the antibody

A
  • Heavy chain is made up of 4 parts: Vh, CH1, CH2 and CH3
  • The light chain is made up of 2 parts: Vl and Cl
  • Disulphide bridges hold the structure together
  • The hinge region provides flexibility to the structure
  • Glycosolation on CH2 to help with interactions
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4
Q

What is the function of the complementary determining region

A
  • Finger-like structures that are responsible for antigen binding
  • On the light and heavy chain
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5
Q

List the functions of antibodies

A
  • Virus and toxin neutralisation
  • Opsonisation + ADCP
  • Complement fixing/ MAC formation
  • Opsonisation + ADCC
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6
Q

What are the different classes of antibodies

A
  • IgG
  • IgD
  • IgA: Dimer of 2 antibodies
  • IgE
  • IgM: 5 monomeric molecules
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7
Q

What different heavy chains does each antibody class use

A
  • IgM - u (mu)
  • IgD - s (delta)
  • IgG - y (gamma)
  • IgA - a (alpha)
  • IgE - e (epsilon)
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8
Q

What are the molecular weights off each antibody type

A
  • IgM - 900k
  • IgD - 180k
  • IgG - 150k
  • IgA - 385k
  • IgE - 200k
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9
Q

What antibody types fixes complements

A
  • IgM
  • IgG
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10
Q

What antibody type can cross the placenta

A

Only IgG

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11
Q

What does the constant region bind to in antibodies

A
  • In IgG it binds top Phagocytes
  • In IgM it binds to mast cells and basophils
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12
Q

What is the main function of IgM antibodies

A
  • Main Ab of primary response
  • Best at forming immune complexes and fixing complements
  • Monomer serves as BCR
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13
Q

What is the main function of IgD antibodies

A
  • The only Ab that isn’t secreted
  • B-cell receptor
  • Indicated mature b cell
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14
Q

What is the main function of IgG antibodies

A
  • Main Ab of secondary responses
  • Neutralises toxins
  • Opsonization
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15
Q

What is the main function of IgA antibodies

A
  • Secreted into mucous, tears, saliva and colostrum
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16
Q

What is the main function of IgE antibodies

A
  • Allergy
  • Anti-parasites
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17
Q

What is class switching

A
  • Only affects heavy chain constant region
  • Allows for different effector function to deal with different pathogens
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18
Q

What are they types of class switching

A
  • Minor class switching - Differential splicing
  • Major Class switching - DNA recombination
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19
Q

How doe step B cell know which class to switch to

A
  • Sensing chemicals around them produced by T helper cells
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20
Q

What is required by Class switch recombination to work on heavy chain switching

A
  • Cytokine signal
  • Switch regions
  • AID and DSB repair proteins
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21
Q

What are the key prerequisite for CSR

A
  • recombination occurs between switch regions
  • Switching can only occur downstream
22
Q

Explain how does Class Switch Recombination works

A
  • When a signal is received to class switch AID and DSB repair proteins are activated
  • The switch regions from the unwanted regions are pulled together
  • This part is cleaved and formed into a Switch DNA circle
  • The VDJ segment is transcribed as normal and the new antibody class is expressed
23
Q

What are the structural differences between secreted IgG snd membrane bound IgG

A
  • Membrane-bound IgG has a hydrophobic transmembrane region and a cytoplasmic region which allows it to be anchored to the cells’ membrane
  • Secreted IgG has a normal tail piece which allows it to be free floating
24
Q

Explain how differential splicing allows creation of secreted and membrane-bound antibodies

A
  • Polyadenylation at 2 different sites allows for the formation of one variant with tailpiece and another with membrane domains
25
Q

Revision: What is the difference between Somatic recombination and differential splicing

A
  • Somatic recombination is the alteration of the DNA itself to induce change in expression
  • Differential splicing are changes made in mRNA to alter the final product, doesn’t affect original DNA
26
Q

What are the stages in B cell development

A
  • Antigen independant stage
  • Antigen dependant stage
27
Q

Describe the formation of a B cell in the bone marrow

A
  • Stem cell becomes Pro-B cell
  • Pro-B cell undergoes D-J and V-DJ recombination to code in the heavy chain variable region - expressed as the mu heavy chain and becomes a pre-B cell
  • Pre-B cell undergoes another V-J recombination to code in the Light chain variable and constant regions and become Immature B cells
  • Once they express IgM and IgD from differential splicing they become mature B cells
28
Q

What happens to B cells when they encounter a pathogen

A
  • Further mature the ability to bind to pathogen to GC
  • Class switch to appropriate effecter regions
  • Become either IgG or IgA
  • Become plasma or memory cells
29
Q

How many unique B cell receptors do we have

A

1 billion unique receptors

30
Q

What gene segments make up the antibody light chain

A
  • V, J and C
31
Q

What gene segments make up the antibody heavy chain

A
  • V, D, J and Cmu
32
Q

How does V (D) J recombination happen to produce unique heavy and light chains

A

Different gene segments are alternatively spliced together to make unique heavy and light chains

33
Q

Where are the gene loci encoding for Ig located in the genome

A
  • There are 2 light chain loci and one for the heavy chain
  • Lamda on chromosome 22
  • Kappa on chromosome 2
  • Heavy chain on chromosome 14
34
Q

Describe the process of VJ recombination

A
  • V and J gene segments are chosen at random
  • The gene segment is transcribed
  • The transcript is then polyadenylated and spliced
  • The transcript is translated to a amino acid sequence
  • The sequence folds to become a protein
35
Q

Describe the process of VDJ recombination

A
  • V, D and J gene segments are chosen at random
  • The gene segment is transcribed
  • The transcript is then polyadenylated and spliced
  • The transcript is translated to a amino acid sequence
  • The sequence folds to become a protein
  • The C segments determines what types of antibody it is
36
Q

What are recombination signal sequences in V(D)J recombination mechanisms

A
  • They are conserved sequences upstream or downstream go gene segments
  • ‘Turns’ consisting heptameter and nonamer with 12 or 23 bp spacer
37
Q

Why do we have the one-turn/two-turn rule

A
  • Recombination only occurs between a segment with a 12bp spacer and a 23bp spacer
  • Prevents V and J fragments from accidentally recombining
38
Q

What other ways does antibody diversity come from

A
  • Junctional flexibility
  • P-nucleotide addition
  • N-nucleotide addition
  • Combinatorial association of heavy and light chains
  • Somatic hypermutation during affinity maturation
39
Q

How is junctional diversity created

A
  • Occurs during VDJ recombination via junctional diversity
  • P and N nucleotide addition before end joining
  • Also produces non-productive rearrangements
40
Q

Describe the mechanism how which VDJ recombination occurs

A
  • RAG 1 and 2 bind the turn ends and form a major hairpin structure
  • Cleavage occurs at the turn ends to produce minor hairpins between the DNA strands
  • More enzymes repair the ends and joins the ends together
41
Q

Describe the hairpin opening and closing mechanism

A
  • the hairpins are repaired by enzymes such as Artemis and endonuclease
  • The enzymes mess around with the free DNA ends by adding or removing nucleotides
  • The ends are joined together by other enzymes
42
Q

How does Artemis cause junctional diversity

A

Artemis chooses at random what nucleotide to cut which changes the nucleotide code for future repairs

43
Q

How does allelic exclusion work

A
  • Two copies of each Ig gene – one from the mother and one from the father
  • In other cells, both genes are expressed
  • Antibody genes different – Only one heavy chain allele and one light chain allele is expressed
  • Order of rearrangement: Heavy>kappa>lambda; 1st allele then 2nd
  • Ensure each B cell makes one type of antibody
44
Q

Describe the Antigen dependant phase of B cells

A
  • T cell activates B cell by presenting pathogen antigen
  • B cells migrate to germinal centre where it undergoes affinity maturation
  • Affinity maturation involves: Clonal expansion, Somatic hypermutation and selection
  • They undergo class switching to be more appropriate for the infection
  • Become plasma and memory cells
45
Q

What are the 2 stages of B cell activation

A
  • T cell dependant phase
  • T cell independent phase
46
Q

Describe the T cell independent stage

A
  • When a pathogen invades some B cells bind to antigens and become partially activated
  • Undergoes clonal expansion
  • Some clones secrete IgM and become the first defence
  • Other clones migrate to lymph nodes awaiting T cell activation
47
Q

Describe how B cells are activated in the lymph nodes

A
  • Requires triple verification process
  • Encounter rte pathogen and internalise antigen
  • Antigen presented on B cell surface
  • Activated by T cell which the same pathogen has activated
  • CD40/CD40L handshake ensures it is a T cell
  • Cytokine signalling is needed as well and the B cell undergoes affinity maturation and class switching
48
Q

Describe the signal transduction pathways present in B cells

A
  • BCR binding > Activation of tyrosine kinase
  • Signal transduction pathway for cell proliferation, differentiation and survival
49
Q

How does affinity maturation occur

A
  • Aims to fine tuner antibody affinity to antigen
  • After clonal expansion, the clones undergo affinity maturation
  • Mutations occur in variable region genes and select for antibodies with the highest affinity for the antigen
  • Takes place in germinal centre of lymph node
50
Q

Describe the selection stage in the light zone

A
  • Follicular Dendritic cells present antigen on the cell surface
  • B cells compete to bind to antigen and present it to Tfh cells
  • Cell with reduced affinity is apoptosed
  • Survived b cells migrate back to the dark zone to repeat processes until affinity is high enough

Microbiology and Immunology - SGUL (Sem 4) (30 decks)

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