L5 - Humoral responses Flashcards

1
Q

Steps in B cell activation and differentiation

A
  1. B cells develop inbone marrow - precursor rearranges immunoglobulin genes to create BCR
  2. cytokines bind and promote survival
  3. migration to lymph nodes to wait to be activated
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2
Q

what are BCRs

A

B-cell receptors are cell surafce Immunoglobulins- Ig

same structure as an antibodyb but has a transmembrane region to tether it

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3
Q

what part of BCRs are involved in intracellular signalling

A

BCR alone cannot generate signal

Immunoreceptor tyrosine-based activation motifs (ITAMs)

on Iga and Igb hetrodimer

phosphorylation of tyrosine residues on activation cause intracellular signal

= P13kinase

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4
Q

what are antibodies made of (chains and regions)

A

2 identical L chains + 2 identical heavy chains

each light chain has:
- 1 variable region VL
- 1 consrant region CL

each heavy chain has:
- 1 variable region VH
- 3 constant regions CH1,CH2 and CH3

hinge protein in between

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5
Q

what is in the Variable regions of the heavy and light chains that binds to antigens

A

hypervariable/complimentatiry-determining regions

V region is made of beta-pleated sheets with loops between them

loops bind to antigens

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6
Q

how many CDRs regions /hypervariable per chain per antigen binding site (remeber 2 identical binding sites per antibody)

A

3

so 6 per side of Y

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7
Q

what is the orginal ‘germline theory’ for number of antibodies produced and why is this wrong

A

1 gene per antibody

not enough genes in genome to code for all possible antibodies

= let alone rest of bodies proteins

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8
Q

what is the answer to limited genome and unlimited antibodies

A

VDJ recombination

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9
Q

what are the 2 chromosones that code for light chains (heavu chain coded for by a different one)

A

lambda and kappa

every antibody is 1 kappa/lambda and 1 heavy chain

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10
Q

describe VDJ recombination - simple

A

light chain composed of smashing togther random gene segments :

1 V gene + 1 J gene

heavy chain made of:

1 V gene + 1 J gene + 1 D gene

constant regions join to the new gene and do not partake in recombination = effector function

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11
Q

what is the role of Recombination site sequnces (RSS) in VDJ recombination

A

short DNA sequnces that flank the regions - V.D and J

site of RAG recruitment and where it cuts

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12
Q

what is the 12/23 rule in VDJ recombination and how does it ensure thinsg work fine

A

There are 12 or 23 bases between RSSs

a 12 RSS can only bind to a 23 RSS

heavy chain V + J genes are ONLY 23 spacers

cannot come together

= ensures inclusion of 12 spacer D gene in the middle

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13
Q

name of the enzyme complex that undergoes somatic recombination

A

V(D)J recombinase

Includes:
1. lymphocyte-specific RAG-1 and RAG-2 endonuclease

  1. Ku70 and Ku80
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14
Q

role of RAG endonucleases

A

bind to RSS serqunces

recruit andother RSS

cleave between RSS and coding sequnce removing the non-chosen segments

= leaves hairpin loop

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15
Q

jucntional diversity mechansim - following on from RSS cleavage by RAG

A
  1. hairpin loops bound by Ku70/80 and recruit artmesis
  2. cleaves hairpin loop leaving open ends
  3. TDT binds - Terminal deoxynucleotide transferase
  4. randomly adds and removes nucleotides to gap until complimentartity found

= junctional diversity at CDR3

  1. DNA ligase heals nick
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16
Q

why is there increased diversity via junctional diversity at CDR3 compared to the other hypervariable regions

A

CDR1 and 2 coded by a single segment - a V region

no junctional diversity can occor here due to no cleavage by RAGs

= CDR3 includes V + J (and a D if the heavy chain)
= chance for random nucleotides to be added at end of gene segments

17
Q

summary of VDJ corecomination: combinatorial diversity + jucntional diversity

A

combinatorial:
multiple V,D and J genes combine
any heavy chain can combine with any light chain to generate antigen-binding site

junctional:
random nucleotides added to ends of genes

18
Q

what can B cells do AFTER encountering an antigen to produce higher affinity antibodies

A

somatic hypermutation

= T cells once matured never change affinity

19
Q

what is somatic hypermutationn and what enzyme is most involved

A

B-cells continously undergo 1 or more amino acid changes in the hypervariable regions

AID enzyme - activation induced cytidine deaminase

20
Q

where does somatic hypermutation take place

A

germinal centres in lymph nodes
1. Light zone:
somatic hypermutation site and rapid division of B-cells

  1. Dark zone:
    affinity maturation - cells compete to identify who has highest affinity
21
Q

Describe affinity maturation in steps (thinkl helper T cells)

A
  1. advantageous mutations in B cells bind and present antigen better
  2. T follicular helper (Tfh) cells produce selecting cytokines
  3. cytokines promote growth and survival
22
Q

B cells can be activated by T-cell dependant or independant pathways - describe the differences

A

B cells always need 2 signals to be activated

dependant - protein antigens:

  1. BCR binds to antigen and presnets it via MHC 2
  2. T cells bind via MHC increasing expressiuon of CD40 leukin

= survival signal

independant - non-protein antigen:
1. antigen binding to BCR
2. binding of PAMP to TLR

= produces NFkB transcription factor

23
Q

why do T-cell independant activations of the B cells look like innate responses

A

No memory cells produced

No affinity maturation or somatic hypermutation

No class switching of antibodies

= short lived plsams cells that cannot clas siwtch or undergo affinity maturation

= T cells provide the signals needd for all these things via cytokines and bidning of CD40

24
Q

term for chaning the Fc region of an antibody

A

class switching

= no change in affinity (Fab) but changes effector functiom

25
how many types of antibodies are there name them
5 IgM IgD IgG IgE IgA
26
put the 5 antibodies in order they are produced in an immune reponse from earliest to latest
IgM / IgD , IgG , IgE , IgA = M more important than D - D is not fully understood
27
what vstrcuture does IgM form and hown is this useful as an early produced antibody
pentamer via J chain and disulphide bonds increases avidity = overall strength of interaction with binding due to 10 biunding sites useful as IgM does not undergo somatic hypermutation due to early produced - maay not be highest affinity
28
why are both IgM and IgD found on surface of mature B cells and both involved in early response
produced from the same mRNA transcript alternative splicng decides which is produced
29
which enzyme is involved in class switching AND somatic hypermutation
AID - Activation Induced cytidine Deaminase
30
how does class switching happen
constant regions of heavy chains have 'switch regions' jsut before them AID cleaves DNA at switch regions = changes antibody isotype
31
role of antibodies
neutralisation opsonisation tagging for NK cells - Nk cell binds to the free fc region of antibody activation of classical complement pathway allergic response with mast cells - IgE
32
explain the allegric reponse with mast cells and IgE
resting mast cells bind with high affinity to IgE bind to antigens and cross link cross link causes release onf granules = histamines and inflammatory cytokines
33
describe the classical complement pathway caused by IgM antibodies
pentameric IgM binds to bacterial surface forms 'staple form' C1q binds to staple form IgM = activates pathway eventually producing C3 convertase