Immunoglobulins and B-Cell Diversity

How many polypeptide chains in an immunoglobulin?

4: 2 identical heavy chains and 2 identical light chains

What is in each heavy chain?

variable region, constant region, Fc region

what is in each light chain

one variable and one constant region

what links the heavy chains

disulfide bonds between cysteines

what is the hinge region

flexible loop that doesn’t fold into a stable structure to support immunoglobulin antigen interactions

What does the protease papain do

cleave the hinge region to produce 2 Fab fragments and one Fc fragment

(True or false) Immunoglobulins bind to antigens directly

True

hypervariable regions aka complementarity-determining regions (CDRs)

flexible loops in the variable region of amino acid sequences that gains diversity through somatic recombination and junctional diversity

epitopes

specific part of an antigen recognized by an immunoglobulin

multivalent antigen

bind to multiple copies of the same immunoglobulin

avidity

overall strength of binding from multiple interactions

What do multivalent antigens promote

antibody clustering, antigen recognition, complement activation, phagocytosis

linear epitopes

recognizes a specific amino acid sequence when the 3D structure is disrupted

conformational epitopes

depend on the antigens complete 3D structure

Unproductive rearrangmensts lead to

apoptosis

Productive rearrangements allow

receptor expression

isotype switching

changes DNA at the heavy chain locus so a different constant region is expressed

What does the constant region Cμ correspond to

corresponds to IgM

What does the constant region Cδ correspond to

corresponds to IgD

What does the constant region Cε correspond to

corresponds to IgE

What does the constant region Cγ correspond to

corresponds to IgG

What does the constant region Cα correspond to

corresponds to IgA

switch regions

lie upstream of the constant region genes and has targets for isotype switching

When and where does isotype switching occur

after B cell development in secondary lymphoid tissue

(True or false) Naive B cells can only express IgM and IgD

True

What does the production of IgM and IgD require

alternative splicing processes a transcript of the μ and δ constant region sequences

What do Naive and memory B cells do

express membrane-bound immunoglobulins

what do plasma cells do

produce and secrete soluble immunoglobulins

What happens if an RNA transcript with a hydrophobic amino-acid sequences that spans the membrane is retained

produce a membrane-bound BCR that is transported through the ER and secretory pathway to the surface

What happens if an RNA transcript with a hydrophobic amino-acid sequences that spans the membrane is removed

it is removed by RNA splicing to produce a soluble antibody secreted by the B cell

When are IgA, IgG, and IgE produced

after activation and isotype switching

what does affinity maturation do

increase antibody affinity by mutating immunoglobulin genes during B cell activation

when does affinity maturation happen

after a primary immune response and contributes to improved secondary responses

what does activation-induced cytidine deaminase(AID) do

deaminates cytosine in DNA to produce uracil

where do mutations occur for somatic hypermutation

hot spots in the hypervariable loops of heavy and light chain variable regions

what does diversification activator (DIVAC) do?

recruit machinery for hypermutation

How does DNA replication repair and mutate

uracil is read as thymine so it converts the original C-G pair to a T-A pair

How does base-excision repair and mutate?

Uracil-DNA glycosylase removes uracil to create an abasic site —> endonucleases remove the site —> short patch repair polymerases replace the missing base which are error prone and can introduce subsititutions

How does mismatch repair and mutate

larger stretch of DNA is excised when U-G mismatch is recognized —> error prone DNA polymerase fills in the gap and introduces mutations

How do activated B cells switch isotypes

AID targets sequences in switch regions upstream of constant region genes —> AID targets cytosines in the donor switch region and the acceptor switch region upstream of the desired isotype —> uracil removal creates abasic sites —> endonucleases or repair pathways process the DNA and enable recombination

What does AID require

single stranded DNA from active transcription

where and when does switching occur

in the germinal centers of secondary lymphoid tissue after helper T cells activate B cells through MHC recognition and cytokines

What cytokine is associated with IgG3 and IgG2a

IFN-γ is associated

What cytokine is associated with IgG1 and IgE

IL-4 is associated

what cytokine is associated with IgA

IL-5 is associated

What cytokine is associated with IgA and IgG2b

TGF-β is associated

What are the major effector functions of antibodies

neutralization, opsonization, classical complement activation, immune cell activation, protection of internal tissue

What does soluble IgM commonly form

pentamer stabilized by disulfide bonds and the J chain

What does IgM do

neutralize antigens, serve as the C-reactive protein in classical-pathway activation

What are characteristics of IgM

10 antigen binding sites with high avidity and low affinity

what does IgD do

activate mast cells and basophils

What does IgA do

protect mucosal surfaces, neutralize pathogens at mucosal surfaces, transport across mucosal epithelial cells

Characteristics of IgA

Monomer or J-chained linked dimer

(True or False) IgG is the most abundant immunoglobulin

True

What does IgG do

neutralization, opsonization, activate classical complement pathway

How many IgG subclasses

4 with differences in heavy chain constant regions and hinge region length

What is special about IgG4

exchange heavy and light chain pairs to produce bispecific antibodies with 2 antigen binding sites

What does IgE do

associates with granulocytes(mast cells, basophils, eosinophils) to trigger degranulation and release of inflammatory mediators to inhibit pathogens and expel them

What type of infections does IgE combat

inhaled or ingested pathogens, like parasites

What is special about IgE

bridges innate and adpative immunity