B Cells/Antibodies Flashcards
Basic Immunoglobulin Structure
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
IgG 1-4
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
IgM
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
IgA 1/2
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
IgE
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
IgD
similar structure to IgG, trace protein
found with IgM in mature B cells
binding of this to B cells results in plasma cell differentiation
K Light Chain Gene Expression
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.
V(D)J recombinase
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.
People who are deficient in the RAG proteins
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.
After the VJ construct is formed
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.
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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.
Heavy Chain Gene Expression
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”
After the VDJ construct is formed
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.
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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.
Imprecision in joining the V, D, and J (orV and J) exons can lead to
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.
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
terminal deoxynucleotidyl transferase; this is called N-region diversity.
The antibody becomes a complete, functional glycoprotein when
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.
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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.
Antibody Class Switching from IgM
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.
Explain why IgM is 1* and IgG is 2*
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.
AID enzyme is activated when
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.
The cytokines produced by the THcell will determine:
which class of antibody the B cell will synthesize.
IL-4
induces class switch to IgE of IgG4
IFN g
induces production of IgG3
Persons who lack functional AID enzyme, CD40, or CD40L cannot
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.
Somatic hypermutation
point mutations in the variable region exon that lead to the production of higher affinity antibodies (affinity maturation) as the immune response progresses.
5 sources of Antibody Diversity
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
