Transcript Antibody Structure and Function

Antibody Isotypes, Idiotypes,
and Allotypes
W. Robert Fleischmann, Ph.D.
Department of Urologic Surgery
University of Minnesota Medical School
[email protected]
(612) 626-5034
Objectives
• Understand the terms isotypes,
idiotypes, and allotypes of antibodies
• Understand the major characteristics of
each isotype and their biologic functions
• Understand the biological and medical
significance of idiotype and anti-idiotype
antibodies
Billy Daniels, age 8 months, is brought to the
clinic by his parents for testing. While he is
healthy at this moment, for the past 3 months,
Billy has had a series of bouts of pneumonia
caused by Streptococcus pneumoniae and
Haemophilus influenzae (type b). He has
contracted these infections despite the fact that
he has had the recommended vaccinations that
have included vaccinations with pneumococcal
conjugate vaccine (PCV) and Haemophilus
influenzae type b conjugate vaccine (Hib) at 2
months, 4 months, and 6 months.
What thoughts do you have about Billy?
The physician suspects that Billy has an
immunodeficiency.
Blood test results:
Total WBC:
7,400/µl
(4,100-10,900/µl)
Differential WBCs:
Neutrophils
55%
(35-80%)
Lymphocytes 35%
(20-50%)
Macrophages
8%
(2-12%)
Eosinophils
1%
(0-7%)
Basophils
1%
(0-2%)
All other blood test results are normal.
What thoughts do you have about Billy?
What additional tests might you wish to run?
Isotypes have different heavy
chains. They are represent
classes of antibody.
Allotypes have the same
constant regions with minor,
but immunologic differences.
Different individuals have
different allotypes.
Idiotypes are antibodies that
recognize different specific
epitopes. Each idiotype is
composed of several
idiotopes or combining sites.
Antibody Isotypes
Antibody Isotypes (Classes)
Five major classes of antibody
1. IgM
2. IgD
3. IgG
Subclasses: IgG1, IgG2, IgG3, IgG4
4. IgE
5. IgA
Subclasses: IgA1, IgA2
Characteristics of the
Antibody Isotypes
Light Chains
Heavy Chains
Other Chains
IgM
Kappa Chain
Lambda Chain
Mu Chain
J Chain
IgD
Kappa Chain
Lambda Chain
Delta Chain
IgG
Kappa Chain
Lambda Chain
Gamma1 Chain
Gamma2 Chain
Gamma3 Chain
Gamma4 Chain
IgE
Kappa Chain
Lambda Chain
Epsilon Chain
IgA
Kappa Chain
Lambda Chain
Alpha1 Chain
Alpha2 Chain
J Chain
Secretory Component
Structures of Different Isotypes (Classes) of Antibodies
The different antibody
isotypes (classes) differ in
1. Number of subunits
2. Number of domains,
3. Number and location
of carbohydrate
chains,
4. Presence or absence
of hinge region,
5. Number and location
of disulfide bridges.
Antibody Isotypes in the Serum
IgM
IgD
IgG
IgE
IgA
No. of monomers
5
1
1
1
1-3
Molecular Weight
900 kD
150 kD
150 kD
190 kD
150 kD
450 kD
600 kD
Serum Conc (mg/ml)
1.5
0.03
13.5
0.0003
3.5
Serum Half-life (days)
5
3
8-23
2.5
6
No
No
Yes
No
No
Placental Transfer
Biological Activity of Antibody
Isotypes
IgM
IgD
IgG
IgE
IgA
++++
-
+
-
-
-
-
-
++++
-
Bacterial Lysis
+++
-
+
-
+
Antiviral Activity
+
-
+
-
+++
Toxin Neutralization
-
-
+++
-
+++
Complement Fixation
Mast Cell/Basophil
Degranulation
Concentrations of Serum Immunoglobulins
Serum electrophoresis showing the mobility and relative amounts
of the five classes of immunoglobulin. IgG is found in the  region
while most of IgM is found in the  region.
IgM
• Monomeric IgM is expressed on the
surface of B cells where it serves the
role of antigen recognition.
• Pentameric IgM is expressed in the
serum (5-10% of total serum Ab).
• Pentameric IgM is secreted into
mucosal secretions.
IgM Structure
• The pentameric structure of IgM is
stabilized by a J chain (joining
chain) and by disulfide bonds.
• Secretory IgM has a secretory
component that assists in transport
across the epithelial membrane.
• Note that for IgM, the pentameric
structure is shown as a flat
molecule (like a hand flat on a
surface). In reality, the disulfide
bonds would all be the same length.
In its binding form, the molecule
would look more like a hand poised
on a surface with only its fingertips
touching the surface. This allows for
multiple interactions with repeated
epitopes on a given antigen.
Biological Functions of IgM
•
•
•
•
•
•
•
•
Good at virus neutralization
Poor at toxin neutralization
Excellent at bactericidal activity
Excellent at causing agglutination of antigens
Excellent at causing precipitation of antigens
Excellent at complement fixation
Does not bind to M Fc receptors
As a monomer, it serves as surface receptor
for antigens on B cells
Medical Considerations of IgM
• Elevated levels indicate a recent infection or other
exposure to antigen
• Does not cross placenta
• Not useful for protecting immunocompromised
individuals (short half-life and lower specificity
than IgG)
• Not useful as blocking antibody to block TNF
production
• Not useful as blocking antibody to block allergens
• Not present in interstitial fluids (too big)
• Can be present in bodily secretions
IgD
• Monomeric IgD is expressed on the
surface of B cells where it serves the
role of antigen recognition.
• IgD is a very minor component of serum
Ig (0.2%)
• Biological Functions: no biological
effector function has been determined
for IgD.
• Medical Consideration: some multiple
myeloma patients over-express IgD.
IgG
• IgG is the predominant antibody synthesized
during a secondary immune response
(anamnestic response)
• IgG is the most abundant class in serum,
constituting 80% of serum Abs.
• Although it cannot be transported across the
epithelium, it is produced in the distal
pulmonary tract and is important in pulmonary
secretions.
• There are four subclasses of IgG.
Four Subclasses of IgG
Note the much longer
Gamma chain on IgG3
IgG Subclasses
IgG1
IgG2
IgG3
IgG4
Serum Conc (mg/ml)
900
300
100
50
Serum Half-life (days)
23
23
8
23
Complement Fixation
+
+/-
++
-
Crosses Placenta
+
+/-
+
+
Binds to M Fc Receptors
++
+/-
++
+
Longer gamma chain
Biological Functions of IgG
•
•
•
•
•
•
•
Good at virus neutralization
Excellent at toxin neutralization
Good at bactericidal activity
Good at causing agglutination of antigens
Good at causing precipitation of antigens
Good at complement fixation
Binds to M Fc receptors
Medical Considerations of IgG
• Crosses placenta providing protection to the
fetus.
• Can mediate hemolytic disease of the newborn
(blue baby syndrome, Rh mismatch).
• Can be used to protect immunocompromised
(gamma globulin).
• Can be used as blocking antibody to block TNF
production (rheumatoid arthritis).
• Can be used as blocking antibody to block
allergens (desensitization to hypersensitivity)
• Not present in most bodily secretions
• Present in interstitial fluids
IgA
• IgA is the predominant Ig in bodily
secretions.
• IgA in blood can be monomeric, dimeric,
or trimeric. When multimeric, the J
chain stabilizes the interactions of the
two or three monomers.
• IgA in bodily secretions is dimeric and
combined with the J chain and with
secretory component.
Secretion of IgA
• Dimeric IgA molecules, stabilized
as a dimer by the J chain is
secreted into the blood by a
plasma cell.
• The dimeric IgA binds to the PolyIg receptor on the basolateral
membrane of an epithelial cell.
• The dimeric IgA, bound to the
Poly-Ig receptor is internalized in
an endocytic vesicle.
• The Poly-Ig receptor is cleaved,
with the larger peptide remaining
associated with the dimeric IgA
as secretory component.
• The dimeric IgA, with secretory
component, is released on the
luminal side of the epithelial cell.
• Secretory component protects
IgA from enzymatic degradation.
Biological Functions of IgA
•
•
•
•
Excellent at virus neutralization
Excellent at toxin neutralization
Good at bactericidal activity
Good at causing agglutination of
antigens
• Good at causing precipitation of
antigens
• Does not bind to M Fc receptors
Medical Considerations of IgA
• Daily production of IgA is greater than any
other Ig.
• B cells that will produce IgA migrate to
subepithelial tissue of most mucosal epithelia
and of glandular epithelia.
• Present in bodily secretions.
• Present at very high levels in colostrum and
present in breast milk. Provides an excellent
level of protection of newborns against
respiratory and intestinal infections.
IgE
• There is very little IgE in the blood
(0.02% of Ig).
• Most of the IgE is bound to Fc
receptors on mast cells in the epithelial
tissue and on basophils in the blood.
Biological Functions of IgE
• Cross-linking of IgE molecules on the
surface of a mast cell or basophil
causes the release of histamine; the
synthesis of prostaglandins,
leukotrienes, and other chemokines; the
production of various cytokines.
• IgE plays a major role in combating
parasitic infections.
• IgE plays a role in combating pulmonary
fungal infections.
IgE-Mediated Release
of Histamine
• The epitope of an antigen
binds to an IgE paratope.
• If another epitope of the
antigen binds to the
paratope of another IgE
molecules, this causes
cross-linking.
• The cross-linking changes
the character of the binding
of the Fc receptors to the
mast cell, initiating a transmembrane signal.
• The mast cell degranulates.
Medical Considerations of IgE
• Individuals who express allergies to certain
antigens over-produce IgE to those antigens.
• This causes a high level of expression of IgE
with the same paratopes (recognize same
epitope on antigen) on given mast cells. This
makes it easier to cross-link two IgE antibodies.
• When antigen is present, many mast cells are
degranulated, resulting in an over-stimulation of
the immune system that is manifested as an
allergic reaction (Type 1 hypersensitivity).
• IgE plays an important role in asthma, too.
Events
Associated
With Mast Cell
Degranulation
Billy Daniels
Antibody levels in the blood:
Serum IgG
<1.0 mg/100 ml
Serum IgA
<1.0 mg/100 ml
Serum IgM <1.0 mg/100 ml
350-800 mg/100 ml
15-50 mg/100 ml
25-75 mg/100 ml
How do you interpret these results?
What additional tests would you wish to run?
Billy Daniels
Lymphocyte stimulation test:
Phytohemaglutinin
136.2
Concanavalin A
47.9
Pokeweed mitogen
0.0
CD19* positive cells: undetectable
How do you interpret these results?
* A marker for B cells.
134.3
48.8
46.5
Billy Daniels
Billy is diagnosed with X-linked agammaglobulinemia
(Bruton’s agammaglobulinemia). This type of
agammaglobulinemia is caused by a mutation in Bruton
tyrosine kinase (Btk). Btk is needed for proliferation and
differentiation of B cells. Btk mutatiions have variable
severity from individual to individual, indicating that
different mutations to Btk cause variable expressivity.
He is given an intramuscular injection of IgG and will
continue receiving these injections every 3-4 weeks for
the rest of his life.
In addition, Billy must be promptly given antibiotics any
time he has a bacterial infection.