Antibody structure and function

Explain the figure

Heavy chains are identical Light chains are identical Light and heavy chains are broken into domains 4 domains in the HC 4 domains in the LC N terminal domains are the variable regions because they dtr antigen binding specificiity All IgG molecules have the same constant region However, different IgG molecules will have different variable regions

What are the monomer Igs?

IgG and IgE

Which Igs are the multimers?

IgM and IgA

What are the classes of antibody?

IgM, IgG, IgA, IgE, IgD

What is the transmembrane anchor for the B cell?

Igb and Iga

Explain secretory IgA

Isotype mostly associated with mucosal secretions Has a system that transports it from tbe body into muscoal secretions Submucosa = lamina propria IgA B cell produces a dimeric IgA antibody Epithelial cells have polymeric Ig receptor that binds to the IgA and the receptor transports it across the cell. As it transports the IgA across the portion of the receptor comes along with it. This portion is called the secretory component and stabilizes the molecule in the mucosa Secretory IgA contains the dimer and the secretory component Secretory component secreted by epithelial cells

How does isotype switching occur?

Isotype switching occurs in response to signals from antigen and activated Th cells Occurs through changing the constant gene segments by deletion of intervening DNA Recognize Ag but also get signals from T helper cells to tell the B cell which Ab to switch to IgA, Ig?

IgM uses what to hold it's pentomeric shape?

J chain

What are IgA and IgM held togethe rby?

J chain

What can T cells recognize?

Linear epitopes

Mature B cells express which Igs?

Mature (naïve) B cells express both IgM and IgD. Only time two different antibody classes are expressed by a B cell Occurs through differential RNA processing

Describe the picutre

Most antigens are multivalent and multiple epitopes Don't have to have one Ab Some antigens are multivalent with repeated epitope, such as a capsular polysaccharide on a bacteria

What are the functions of antibodies?

Neutralization Opsonization Complement activation Antibody dependent cell mediated cytotoxicity

What is gammaglobulin

Purified Ab or Ig

Explain recombination activating gene proteins and junctional diversity

Recombination activating gene (RAG1, RAG2) proteins Essential for T cell receptors and B cell Ig Bring VDJ gene segments together and cuts DNA Holds together until repair done Cutting isn't precise, lots of wobble At the site that it cuts it's a palindromic sequence On either side of the V, D or J sequences there are RSS sequences RSS sequecnes helps guide splicing that occurs RSS sequences contain palindromic sequences

Explain the picture on the back

Resting B cell has membrane Ig (Ab) on its surface Resting B cell will encounter an antigen and will recognize the surface proteins or epitopes and the B cells will differentiate into plasma cells Plasma cells are Ab producing cells

What are the events that determine specificity independent of Ag?

Selection of VDJ gene segments of heavy chain and VJ gene segments of light chain Junctional diversity P-nucleotides imprecise cuts N-nucleotides Selection variable region gene segments of heavy and light chains

explain the picture

Sequence of events D and J are combined first and the DNA sequence between them is removed Then segment between V & D is removed So randomly pick a V, D and J and that will confer a specific specificity Encodes the variable region of the HC Same thing occurs with the LC but only with two segments All this splicing occurs independent of antigen and is by random chance C = constant region

Explain the picture

Switch regions tell molecular mechanisms where to cut to produce different immunoglobulin's (Ig) Deletes part of chromosomal DNA Downstream we can further process it to create a different Ig (IgA -> IgE) but we can't go back because we've eliminated the DNA Occurs anywhere and depends on T cell cytokines that are present Switching occurs based on Ag interaction and T cell signals Specificity remains the same (different constant regions, but not a different variable region) IgG1 crosses placenta

Explain the picture

The hotspots were interactive with each other and were associated with Antigen binding site 3 areas in the light chain that are very variable between different people These variable LC regions combined with the variable regions on the HC determine specificity Constant regions of the IgA1 are the same and the constant region of the IgA2 are the same but are different between the two

What are the multiple types heavy chains?

a, ∂, e, g, µ

What is somatic hypermutation dependent on?

antigen

What do both heavy and light chain combined determine?

binding specificity

What are P-nucleotides

derived from palindromic sequences with RSS

What does T nucleotide and P-nucleotides add?

diversity

where does somatic hypermutation occur?

germinal centers of lymphoid tissues in response to antigen and activated Th cells

What is the antigen binding site composed of?

heavy and light chain segments Variable regions of each chain form this site

what does Igb and Iga do?

helps transmit signal to cytoplasm when antigen binds to it

What do T cells recognize?

linear epitopes and peptides

What do B cells recognize?

linear, conformational and discontinuous epitopes

Explain the B cell receptor

monomeric with membrane component

Ig variable regions are encoded by?

multiple genetic loci

What does antibody recognize?

native antigen conformational and discontinuous epitopes linear epitopes

What are N nucleotides?

non-template nucleotides added randomly by terminal deoxynucleotide transferase

Explain linear epitope

primary amino acid sequence could be a peptide or denatured protein

What event for antigen specificity is dependent on antigen?

somatic hypermutation which helps fine tune the Ag response

B cells can change their isotype but not their..?

specificity

what does somatic hypermutation affect?

specificity

How many different subclasses for IgA?

2 IgA1 IgA2

Experimenters found how many hypervariable regions?

3

What are the differnet subclasses for IgG?

4 IgG1 IgG2 IgG3 IgG4

What is the basic fuctional component of antibody classes?

A monomer of 2 heavy and 2 light chains

Which of the heavy chains have multiple subclasses?

a and g

Explain B cell receptor anchoring.

Antibody monomer has transmembrane anchor

Explain antibody function

Antibody specifically binds to an antigen and targets its destruction

Explain germinal centers

B and T cells are here together with folicular dendritic cell B cells are proliferating because to T cells interacting with them Fine tuning specificity of Ab to the Ag and B cell is also proliferating as well

Where is the J chain synthesized?

B cell

Where is Ig synthesized?

B cells

What happens if point mutation increases affinity to antigen?

B cells with these mutations are preferentially selected

Explain the picture

Differential RNA processing generate IgM and IgD encoding mRNAs in the same B cell VDJ rearrangement which is upstream of the Cµ and C∂ region Cµ and C∂ encode the constant region of the heavy chain for IgM and IgD respectively At the beginning we get transcription all the way through from VDJ to C∂ To make IgM we have to remove C∂ We take out the intermediate sequence between the MCpAµm and c∂ and the C∂ is degraded Same thing happens with IgD Different Ig depends on where it cuts

Explain the figure

Digest with pepain and you get 2 Fab regions and one Fc region (fragment crystalize for x-ray crystallography) Each Fab fragment has 1 antigen binding site Hangy down things on Fab are disulfide bridges that are called hinge regions Pepain digests above the hinge region Pepsin digest below the hinge region Has two antigen binding sites F(ab)2 Fc receptor on macrophages will recognize the gamma chain portion of heavy chain

Explain IgA

Dimer with a J chain

explain the picture

We can respond to any antigen possible When we thought that it was 1 gene 1 protein, we would have to have 98% of our genome coding for Ig for this to work Dogma not true LC has several regions with V and J segments HC has V D and J gene segments Random assortment of different gene segments confers different specificity This occurs at the DNA level not the RNA level J segment and J chain are not the same thing

what do point mutations result in?

amino acid changes

what is native antigen?

bacterial cells and whole proteins

What occurs if the mutation reduces the affinity?

cell line dies

What does the Th cell tell the B cell?

the specific isotype it needs to produce

discontinuous peptides could be recognized where?

two different binding sites

Rearrangement of variable region gene segments forms what?

variable region

Point mutations occur where

variable regions

Can B cells express different antibody isotypes without changing antigen specificity?

yes

What happens when you change variable regions of an antigen binding site?

you change specificity