5. Humoral responses Flashcards
what cells are involved in the humoral response
B cells and antibodies
where do b cells develop and mature?
the bone marrow
what is the b cell receptor?
a surface immunoglobulin with a transmembrane region and attached to a signalling domain
the same Ig the b cell will secrete
when b cells first encounter the antigen what is phosphorylated?
Src family kinases to activate downstream signallinig pathways
how does a b cells change in morphology once activated?
a bigger nucleus for replication
proteins and machinery to make antibodies
general antibody structure
2 identical light chains
2 identical heavy chains
linked with a disulphide bond
where does papain cleave antibodies?
above the disulphide bond leaving 2 FAB fragments and 1 Fc region
where does pepsin cleave antibodies?
below the disulphide bond leaving 1 FAB fragment and 1 Fc fragment
what does each light chain consist of?
one variable region
one constant region
What does each heavy chain consist of?
one varible region
multiple constant region domains
what is the function of the hinge region ?
allows flexibility when binding to multiple antigens
what are the hypervariable regions?
exposed loops on the outside of the antibody that bind to antigens
they are hypervariable to bind to a diverse range of antigens
What are the framework regions?
a series of B sheets to provide structural integrity to the antibody and keep the variable regions in place
where are the 3 hypervariable regions encoded?
on a single v-region exon
how do we generate a diverse range of antibodies?
somatic recombination of seperate gene segments
what gene segments do light chains have?
variable and joining and a constant region
what gene segments do heavy chains have?
variable, diversity and joining and 1 of 5 constant regions
what is the leader peptide?
a signal sequence that directs the Ig to secretory pathways
what are the 2 light chain gene loci?
lambda and kappa
only 1 is used in each antibody
what heavy chains can light chains combine with?
any heavy chain
what flanks each gene segment?
Recombination signal sequences (RSS)
why are RSSs highly conserved?
they prevent rearrangment with the same cluster
eg a j cant join to a j
v cant join to v
what is the 12/23 rule with signal sequences?
a 23 RSS can only bind with a 12 RSS
prevents joining of different segments of the same type and ensures D region is included in heavy chain recombination
what enzymes are in the VDJ recombinase enzyme complex?
Lymphocyte specific RAG-1 and RAG-2 endonuclease
Artemis:DNA-PK
DNA ligase 4
what do RAG1 and RAG2 do?
binds to an RSS region and recruit another RSS region to it
then cleaves and loops out the DNA between the 2 RSSs
Artemis:DNA-PK does what in double-stranded break repair?
opens the hair pin formed during cleavage
what does terminal deoxynucleotide transferase do in double-strand break repair?
randomly adds nucleotides to the gap
what does DNA ligase do in double-strand break repair?
joins the new DNA backbone together making a continuous piece of DNA
how do B cells generate diversity?
combinatorial diversity
junctional diversity
somatic hypermutation
what is combinatorial diversity?
differnet VDJ combinations
different heavy chain light chain combination
what is junctional diversity?
imprecise addition and removal of nucleotides in recombination
what needs to happen for somatic hypermutation?
encountering the antigen
CD4 T cells
what does somatic hypermutation do?
Increases the mutation rate in the antibodies to make variable regions fit the antigen better
what enzyme initiates the hypermutations?
Activation-induced cytidine deaminase (AID)
what is affinity maturation ?
selects for the survival of the antibodies that have the highest affinity for the antigen
what is a disadvantage of hypermutation?
some mutations can be detrimental and prevent function
these b cells are apoptosed
how are the affinity matured cells selected?
the b cells with the better antigen presentation as a result of antigen binding receive enhanced survival signals from CD4 T cells
what secondary signal is needed for activation in a thymus dependent antigen?
the CD40 receptor and MHC2 on the B cell binding to a CD40L and TCR on a CD4 helper cell
(1st signal is BCR binding)
what secondary signal is needed for activation in a thymus independent antigen?
binding of a common microbial part to a TLR
(1st signal is BCR binding)
what determines antibody effector function?
the Fc portion
what functions do antibodies have?
neutralisation
opsonisation
antibody mediated cellular cytotoxicity
trigger mast cell activation
activation of complement
where does class switching occur?
at the genome level
where are the Fc genes located?
downstream of the VDJ regions
what is IgA functions?
secreted across mucosal membrane
IgM
Pentamer
early in response
no hypermutation so more binding sites
As IgM and IgD are derived on the same pre-mRNA transcript what determines which is expressed?
alternative splicing of the mRNA and polyadenylation
What does AID do?
cuts out switch regions at the 5’ end to loop out other heavy chain genes
what process is AID involved in?
Class-switching and Double-strand break repair mechanisms
what does an AID deficiency result in?
No ability to class switch or affinity mature
Leads to Hyper IgM type 2 immunodeficiency
what is the dynamics of the different antibodies in the immune response?
Initially IgM dominates and then as affinity maturation occurs IgG dominates
somatic hypermutation means affinity increases
what determines which class the antibody switches to?
the cytokines secreted by T helper cells
Which antibodies are stimulated by IL-4?
IgE and IgG1
Which antibodies are stimulated by IFNy?
IgG3 and IgG2a
Which antibodies are stimulated by TGFb?
IgA and IgG2b
Which antibodies are stimulated by IL-21?
IgA, IgG3 and IgG1
Which antibodies are stimulated by IL-5?
IgA and IgG1
where do you find IgG and IgM?
in the circulation
where do you find IgA?
in mucosal sites like GI tract and Respiratory tract
also breast milk
where do you find IgE?
directly under the skin
in mouth and nose
lining external surfaces
what is antibody neutralisation?
bind to viruses, bacteria and toxins to prevent interaction with receptors in the cells
what antibodies mainly carry out neutralisation?
IgG and IgA
what is antibody opsonisation?
Coating the bacteria with the antibodies and then binding to the pathogen surfaces to trigger phagocytosis by crosslinking Fc receptors
what antibodies carry out opsonisation?
IgA, IgG1, IgG3 and IgG4
what does IgE opsonisation look like?
binding to parasites and eosinophils Fc receptors crosslinking to trigger degranulation
what is Antibody-dependant cellular cytotoxicity?
Recognising Viral proteins on host cells and bind Fc receptors on NK cells
crosslinking of NK cell Fc receptors causes degranulation and killing of host cells
what antibodies are involved in Antibody-dependant cellular cytotoxicity?
IgG1 and IgG3
what is antibody sensitisation of mast cells?
cross-linking of IgE triggers degranulation and inflammatory mediators like histamine
allergy response
how do antibodies activate the complement cascade?
IgM or IgG binding to the antigen
C1q then binds to multiple IgG or 1 IgM which can then produce a C3 convertase
