First Aid, Chapter 1 Immune mechanisms: Immunoglobulins (Ig) Flashcards
What is the earliest cell in B-lymphocyte lineage that produces Ig?
pre-B lymphocyte
How much Ig is produced by adults daily?
2-3 g
What is the basic structure of Ig? what kind of molecule is it? How many chains are there? What kind of bond connects the chains? How many Ig domains are in each chain? How many amino acids is Ig made of?
The Ig molecule is a polypeptide heterodimer composed of two identical light chains and two identical heavy chains connected by disulfide bonds (Figure 1-5). Each chain consists of two or more Ig domains, which are compact, globular structures of approximately 110 amino acids containing intrachain disulfide bonds.
How many domains are there in the heavy chain constant region of each Ig type?
The constant (C) regions of IgG, IgA, and IgD consist of only three CH domains. In IgM and IgE, the C regions consist of four CH domains.
What are the light chains of Ig? What region are they identified by? What chromosomes are they encoded on? Can an Ig molecule have one of each light chain?
κ and λ are identified by their C regions. κ is encoded on chromosome 2 and λ is encoded on chromosome 22. An Ig molecule has either κκ (60%) or λλ (40%) but never one of each. An individual B lymphocyte will produce only κ or λ chains but never both.
Where does papain cleave? Where does pepsin cleave? What is the resultant fragment? Can they both cross-link and bind? Can they fix complement or bind to the Fc receptor on the cell surface?
Papain cleaves Ig above the hinge (as seen in Figure 1-5) and results in two Fab (antigen binding) fragments and one Fc (crystallizable) fragment. Pepsin cleaves Ig below the hinge at multiple sites and produces F(ab′)2, which contains interchain disulfide bonds, and exhibits two antigen-binding sites. F(ab) can bind but not cross-link; and F(ab′)2 both binds and cross-links. Neither F(ab) nor F(ab′)2 will fix complement or bind to the Fc receptor on the cell surface.
What are CDRs? Where are they located? How many amino acids are in each? How many CDRs are there? What does the CDR allow for? Which CDR is the most variable?
VL and VH form the antigen-binding sites that consist of complementarity-determining regions (CDRs), 10 aa that account for antibody diversity. There are three CDRs in each V region, CDR3 is the most variable and has the most extensive contact wiht the antigen.
How can light chains be used to diagnose and type B-lymphocyte lymphomas?
The ratio of κ-bearing lymphocytes to λbearing lymphocytes can be used as an indication of clonality and is, therefore, useful in diagnosing and typing B-lymphocyte lymphomas.
Where does omalizumab bind?
Cε3
What is the most variable part of the Ig molecule?
CDR3
Which terminus are CH and CL located at?
The c-terminus of the Ig molecule.
Which part of the constant region mediates effector funcion?
CH by binding to Fc receptors or binding complement.
Why is glycosylation of Igs important?
Glycosylation of Igs is important in maintaining their structural stability and effector functions.
What are deglycosylated IgGs unable to do?
Deglycosylated IgG cannot bind FcγRs and C1q effectively and therefore is unable to trigger antibody-dependent cell-mediated cytotoxicity (ADCC) and complement activation.
Decreased glactosylation of Ig is associated with which diseases?
Rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Crohn’s disease, and tunerculosis (TB).
What is the glycosylation site in IgG?
Human IgG has one conserved glycosylation site in the Cγ2 domain (asparagine-297).
What is the benefit of sialic acid enrichment in IVIG?
Sialic acid enrichment in intravenous immunoglobulin (IVIG) preparation significantly increases its anti-inflammatory activity.
What molecules belong to Ig superfamily?
TCR, MHC molecules, CD4, CD8, CD19, B7-1, B7-2, Fc receptors, KIR (killer cell immunoglobulin-like receptor), and VCAM-1.
What is the shortest half-life of all IgG subclasses?
IgG3
Which Ig isotype fixes complement most efficienty?
IgM
What is rheumatoid factor?
An antibody against the Fc portion of IgG. RF is most commonly IgM, but can also be any other isotype.
What conformations of antigens do Ig recognize?
They can recognize highly diverse antigens through linear and conformational determinants found in various macromolecules (i.e., proteins, polysaccharides, and lipids).
What conformation of antigens do TCRs recognize?
TCRs only recognize linear determinants of peptides presented by MHC molecules.
What is the first Ig to be produced after birth?
IgM
What is the first Ig to reach adult level?
IgM
What cells produce IgA in the GI tract?
IgA is produced by plasma cells in the lamina propria.
How is IgA transported across the mucosal epithelium?
By poly-Ig receptor (transcytosis).
What cells synthesize the poly-Ig receptor?
Mucosal epithelial cells.
Where are poly-Ig receptors expressed?
On the basolateral surface of mucosal epithelial cells.
How does IgA reach the luminal surface in the GI tract?
Once inside the epithelial cell, IgA bound to the poly-Ig receptor is actively transported in vesicles to the luminal surface.
How many heavy chains are in each of the immunoglobulin isotypes?
IgG, IgA, and IgD have 3 heavy chains. IgM and IgE have 4 heavy chains.
What are the serum levels of each of the immunoglobulin subclasses?
IgG total 700-1500 IgG1 430-1050 IgG2 100-300 IgG3 30-90 IgG4 15-60 IgA total 60-450 IgA1 90-325 IgA2 80-290 IgM 50-250 IgE 0.0015-0.2 IgD 0.3-30
What are the half-lives of all the Ig isotypes?
IgG1, IgG2, IgG4 - 23 days IgG3 - 8 days IgA1, IgA2 - 6 days IgM - 5 days IgE - 2 days IgD - 3 days
Which cytokines induce class switching for each Ig isotype?
IgG1 -IFNγ, IL-4 IgG2 - IFNγ, TGFβ IgG3 - IFNγ IgG4 - IL-4, IL-13 IgA1 and IgA2 - TGFβ, IL-5 IgE - IL-4, IL-13
Which Ig’s fix complement and how well?
Best - IgM
moderate - IgG1, IgG3
mild - IgG2
none - IgG4, IgA, IgE, IgD
Which IgG subclass is transported across the placenta the least amount?
IgG2.
What is the function of IgG1?
Th1 response, opsonization Best for ADC
What is the function of IgG2?
Antipolysaccharide Ab
What is the function of IgG3?
Opsonization
Which IgG subclass is the last to reach adult levels?
IgG2
What is the function of IgG4?
Antipolysaccharide Ab, Th2 response
Which IgG subclass is elevated in immunotherapy/
IgG4
Where are the IgA subclasses found?
IgA1 in serum and respiratory tract IgA2 in lower GI tract
What is the function of IgM?
isohemagglutinin and rheumatoid factor
What is the only Ig to bind mast cells?
IgE
What is IgD a marker of?
B cell maturation.
What kind of bonding does Ig capture antigens through?
noncovalent reversible binding through the Ig V regions.
Where do somatic mutations lead to changes in Ig?
V regions, not C regions.
Where does class switch recombination lead to changes in Ig?
C regions, but not V regions
What does alternative splicing change Ig to?
Changes from transmembrane to secretory form.
What is the secreted form of all the Igs (ie - monomer, dimer, etc)?
IgG - monomer (150 kda) IgA - monomer (170), dimer (390) IgM - monomer (180), pentamer (900) IgE - monomer (190) IgD - monomer (180)
Name 6 ways diversity is achieved in Igs and TCR?
1) antigen-binding site variability in Ig and TCR
2) germ-line variation
3) combinational diversity (somatic recombination)
4) Junctional diversity
5) somatic hypermutation
6) receptor editing
What is germ-line variation with regard to TCR and Ig?
variation in the inherited germ-line V, D, and J elements in Ig and TCR
What is combinational diversity (somatic recombination)? How does it affect B and T lymphocytes?
different V, D, and J segment rearrangement in developing B and T lymphocytes resulting in moderate level of receptor diversity
What is junctional diversity?
random nontemplated addition or removal of nucleotide sequences at junctions between V, d, and J regions resulting in extensive somatic variability in immune receptors
What is somatic hypermutation?
point mutations in V regions of Ig in rapidly dividing B lymphocytes causing increase or decrease in antibody affinity
What is receptor editing?
changing Ig specificity that have self reacting antibodies
Which somatic recombination process introduces the greatest diversity in immune receptors and which enzyme is important in this process?
Junctional diversity and TdT.
How is class switch recombination achieved (isotype switching)?
Change in heavy-chain C regions, with same V region at the gene locus . Switch in Ig isotype from IgM or IgD to IgG, IgA, or IgE
What is affinity maturation?
Process of somatic hypermutation and selective survival of B lymphocytes that produce highaffinity antibodies, results in increased Ig affinity.
What is alternative splicing?
Splicing at different locations in the 3′ of C region exons resulting in production of membrane bound or secreted Ig forms.
Splicing at different location of the C-terminal of the IgM gene resulting in production of IgD from the same RNA transcript with IgM. Not conventional class switch
What are the FcR and CD names for IgG? Are they high or low affinity?
FcγRI CD64 High FcγRIIA CD32 Low FcγRIIB CD32 Low FcγRIIIA CD16 Low FcγRIIIB CD16 Low
What cells carry the FcγRI and what is its function?
Macrophages, neutrophils eosinophils
Phagocytosis
What cells carry the FcγRIIA and what is its function?
Macrophages, neutrophils, eosinophils, platelets
Phagocytosis (poor)
What cells carry the FcγRIIB and what is its function?
B lymphocytes
Feedback inhibition of B lymphocytes
What cells carry the FcγRIIIA and what is its function?
NK cells, macrophages ADCC
What cells carry the FcγRIIIB and what is its function?
Neutrophils, macrophages, eosinophils
Phagocytosis (poor)
What are the Fc receptors for IgE? What is the CD name?
FcεRI High
FcεRII CD23 (not 32!)
Low
What cells carry the FcεRI and what is its function?
Mast cells, basophils, eosinophils
Degranulation, ADCC
What cells carry the FcεRII and what is its function?
Neutrophils, eosinophils, monocytes
Unknown
