B Cells/Antibodies

Question 1

Basic Immunoglobulin Structure

Answer 1

2 heavy and 2 light chains

• H- g, m, d, e, a
• L- k, l

Fab = fragment antigen binding site
has hypervariable region which determines the complement

Fc = fragment constant region
AA sequence varies little, carbs here, recognized by phagocytes, NK, and granulocytes, binds to C1q and CRP (IgG/IgM), determines the CLASS OF ANTIBODY

Question 2

IgG 1-4

Answer 2

longest life, most common,
activates complement, opsonization, ADCC, toxin/virus neutralizer, agglutination

valence of 2

CROSSES PLACENTA

major part of secondary immune response

When elevated in people >6mos indicative of vaccination, past infection or chronic infection

Question 3

IgM

Answer 3

First class made in IR (PRimary)

5 IgG-like molecules linked by J

valence 10

classical complement, agglutinator

Elevated levels indicate acute infection

only class produced by a fetus

blood typing

Question 4

IgA 1/2

Answer 4

serum and seromucus secretions

dimeric (valence 4)

secretory component from mucous epithelium protects sIgA from proteases

sIgA protects MALT from microbial infection and toxins by interfering with their adherence

serum conc of 1.8 mg/mL

Question 5

IgE

Answer 5

similar structure to IgG, trace protein

binds to basophils, mast cells and eosinophils with FceR to cause degranulation and release of histamine (inflammation)

Helminths

Immediate hypersensitivity, asthma, hayfever

Question 6

IgD

Answer 6

similar structure to IgG, trace protein

found with IgM in mature B cells

binding of this to B cells results in plasma cell differentiation

Question 7

K Light Chain Gene Expression

Answer 7

contains about 40 variable (V) region exons and 5 joining(J) region exons. Any V region exon can join with any J region exon. During this process, all intervening introns and unused exons are looped out, excised from the DNA, and discarded by the B cell. The resultant “VJ construct” codes for the variable region of the kappa light chain.

Question 8

V(D)J recombinase

Answer 8

responsible for looping out and excising the DNA separating the V and J exons. Two important components of the VDJ recombinase are the RAG-1 and RAG-2 enzymes.

Question 9

People who are deficient in the RAG proteins

Answer 9

cannot make functional antibodies (or TCR molecules on T cells), and suffer from a form of severe combined immunodeficiency (SCID)that leaves them highly susceptible to all kinds of infections.

Question 10

After the VJ construct is formed

Answer 10

B cell transcribes an exact copy of the rearranged DNA, yielding a primary RNA transcript. The primary RNA transcript still contains unused exons and introns.

Question 11

hmmm

Answer 11

The unused exons and introns are spliced out of the primary RNA transcript to generate a mature messenger RNA (mRNA) molecule. During splicing, the kappa constant region exon(C)is brought adjacent to the VJ construct. The mRNA is then translated into a complete kappa light chain protein by ribosomes within the B cell’s rough endoplasmic reticulum.

Question 12

Heavy Chain Gene Expression

Answer 12

The V(D)J recombinase, with its RAG-1 and RAG-2 enzymes, is required for heavy chain DNA rearrangement. The rearrangement takes place in two steps: 1) formation of the “DJ” construct shown below, followed by formation of the “VDJ construct”

Question 13

After the VDJ construct is formed

Answer 13

the B cell transcribes an exact copy of the rearranged DNA, yielding a primary RNA transcript. The primary RNA transcript still contains unused exons and introns.

Question 14

idk

Answer 14

The primary RNA transcript can undergo alternative RNA splicing, which brings either the delta () or mu () constant region exon adjacent to the VDJ constructin the mature mRNA. If the exon is placed next to the VDJ construct, then the antibody will be an IgD. If the exon is placed next to VDJ, then the antibody will be an IgM. In this way, alternative RNA splicing enables a B cell to express both IgM and IgD antibodies on its surface at the same time. Both classes of antibodies will have the very same antigen specificity because they share the same VDJ construct. The VDJ construct codes for the variable region of the antibody heavy chain polypeptide.The mRNA is translated into heavy chain polypeptide by ribosomes in the cell’s rough endoplasmic reticulum.

Question 15

Imprecision in joining the V, D, and J (orV and J) exons can lead to

Answer 15

loss or gain of nucleotides, which in turn may result in amino acid sequence changes at the junction sites. This imprecision leads to junctional diversity and contributes to the overall variability in antigen specificity displayed by antibody molecules.

Question 16

small sets of nucleotides not encoded by the DNA template can be inserted at the V-J or V-D-J junctions by the enzyme

Answer 16

terminal deoxynucleotidyl transferase; this is called N-region diversity.

Question 17

The antibody becomes a complete, functional glycoprotein when

Answer 17

two heavy chains combine with two light chains, the chains are connected by disulfide bridges, and carbohydrates are added to the constant regions of the heavy chains.

Question 18

???

Answer 18

The complete antibody molecule is attached to the surface of the B cell via a C-terminal polypeptide tail inserted in the B cell membrane. When the B cell differentiates into a plasma cell, the polypeptide tail is not translated, and so the antibody is secreted rather than being expressed on the plasma cell surface.

Question 19

Antibody Class Switching from IgM

Answer 19

activation-induced cytidine deaminase (AID) is required

This enzyme recognizes “switch regions” that precede all the heavy chain constant region exons except Cd

causes unused constant region exons and intervening introns to loop out and be excised from the DNA. Consequently, once a B cell has class-switched from IgM to one of the down-stream heavy chain isotypes, it can never again synthesize IgM because the mu constant region exon has been lost.

Question 20

Explain why IgM is 1* and IgG is 2*

Answer 20

once a B cell has class-switched from IgM to one of the down-stream heavy chain isotypes, it can never again synthesize IgM because the constant region exon has been lost. This phenomenon explains why IgM is synthesized during the primary immune response, but other classes, such as IgG, predominate during the secondary response.

Question 21

AID enzyme is activated when

Answer 21

the B cell receives “help” from TH cells in the form of receptor-ligand interactions and cytokines. The most important receptor-ligand interaction for class-switching occurs between CD40 on the B cell and CD40L (CD154) on the TH cell.

Question 22

The cytokines produced by the THcell will determine:

Answer 22

which class of antibody the B cell will synthesize.

Question 23

IL-4

Answer 23

induces class switch to IgE of IgG4

Question 24

IFN g

Answer 24

induces production of IgG3

Question 25

Persons who lack functional AID enzyme, CD40, or CD40L cannot

Answer 25

class-switch from IgM to other isotypes. These individuals suffer from an immunodeficiency disease called hyper-IgM syndrome and are prone to certain infections and autoimmune diseases.

Question 26

Somatic hypermutation

Answer 26

point mutations in the variable region exon that lead to the production of higher affinity antibodies (affinity maturation) as the immune response progresses.

Question 27

5 sources of Antibody Diversity

Answer 27

Random recombination of VJ (light chain) or VDJ (heavy chain) exons

Random combination of heavy chains with light chains

Somatic hypermutation in response to antigen stimulation, mediated by the AID enzyme

Junctional diversity

N-region diversity mediated by terminal deoxynucleotidyl transferase