Antibody Structure and Function Flashcards
Basic composition of antibodies
Glycoproteins
Where are antibodies found?
Surface of B cells
Produces by plasma cells and released into blood serum and secreted fluids like saliva and milk
What do antibodies activate?
The classical complement pathway
3 major functions of antibodies + 3 other roles
- Opsonins to enhance phagocytosis
- Neutralize toxins and viruses by binding to the antigen
- Activating complement
- act as antigen receptors for B cells
- Mast cell degranulation (mainly IgE)
- Antibody dependent cell mediated cytotoxicity (ADCC)
5 classes of antibody?
IgG, IgM, IgA, IgE, IgD
Heavy chain of IgG
γ
Heavy chain of IgM
μ
Heavy chain of IgA
α
Heavy chain of IgE
ε
Heavy chain of IgD
δ
What are the 2 classes of light chains?
κ and λ
What holds the light chains together?
disulphide bonds and non-covalent interactions
What are globular domains?
Sections of about 110 amino acids held together by intrachain disulphide bonds
each light chain has 2 domains while the heavy chain has 4 domains + the hinge region
What is the V region
The variable region present on both the light and the heavy chains
- variable AA sequence
- where antibody interacts with antigen
Both heavy and light chains have 1 V region
What is the C region?
Constant region present in both the heavy and the light chains
- constant AA sequence
- the constant region of the heavy chains is where the effector activity occurs
Light chains have 1 C region. Heavy chains have 3 or 4
Which 2 classes of antibody have a J chain?
IgM and IgA
Which 2 classes of antibody have subclasses of the heavy chain
IgG and IgA
What are the IgA heavy chain subclasses?
α1 and α2
What are the IgG heavy chain subclasses?
γ1, 2, 3, and 4
Which is the more common form of light chain?
the kappa light chain is the default
What can affect antibody stability and interactions with other proteins ?
Glycosylation
Which three regions within the V region form the antigen binding site?
The hypervariable regions
Sequences rich in what amino acid constiture the hinge region?
Proline
What 3 classes of antibody have a true hinge region?
IdD, IgA, IgG
What domain of IgM and IgE has hinge region like properties
Ch2 domain (additional heavy chain domain) -even though it lacks proline rich sequences
Differences in which region os the antibody determines half like, distribution, complement fixing, and Fc receptor binding?
Heavy chain constant region
What is the Fc region?
The fragment crystalizable region
What does proteolysis of IgG with papain yield?
2 identical Fab regions and 1 Fc fragment
What does proteolysis of IgG with pepsin yield?
Divalent F(ab’)2 fragment and a pFc’ fragment
What does reduction of IgG with mercaptoethanol yield?
2 heavy chains and 2 light chains
Antibodies produce what kind of response when injected into other species?
an antibody response
What is an isotypic determinant and where is it located?
Located in the constant region
Define heavy chain classes/subclasses and light chain classes/subclasses within a species
What is an allotypic determinant and where is it located?
Located in the constant region
Can vary from individual to individual (ie. non species specific)
What is an idiotypic determinant and where is it located?
Located in HC and LC variable regions
-both the hypervariable and framework regions
Defined by the unique amino acid sequences that determine antibody specificity
What occurs during opsonzation?
interactions between the Fc region and the Fc receptors on phagocytes promotes phagocytosis
What occurs during antibody dependent cell mediated cytotoxicity?
the antibody acts as a receptor to enable recognition and killing of target cells by Fc receptor bearing NK cells
What occurs during transcytosis?
Passage of antibody across epithelial layers to mucosal surfaces
-only certain classes, primarily IgA (IgM can as well)
What effector function is unique to IgE?
Mast cell degranulation
Which class of antibody can cross the placenta?
IgG
Each light chain weighs about…
25 kDa
Each heavy chain weighs about..
50 kDa
What is the structure of IgG?
2 identical 50 kDa γ and 2 identical 25 kDa κ or λ chains
How many subclasses of IgG heavy chains are there?
4 in humans
-each has a unique biological property
What are the 5 main roles that IgG has?
- Neutralize
- Opsonize
- activate classical complement pathway
- mediate ADCC reactions
- Crosses placenta during pregnancy
What is the structure of IgM and how does it differ between the expressed form and the secreted form?
Expressed as a monomer (2 μ chains, 2 κ or λ chains) on B cells
secreted by plasma cells as a pentamer containing a J chain
What is the most abundant antibody in the blood serum and interstitial fluid?
IgG
How many (of the 10 Fab regions) on IgM are able to interact with large antigens at a time?
5
IgM is the first immunoglobulin class made by…? (2 cases)
Newborns and during a primary immune response
The presence of the J chain in IgM allows it to…?
be transported across epithelial mucosa
-not as effectively as IgA
What are the 2 main function of IgM?
- Agglutination of particulate antigens
2. Activator of the classical complement pathway
Structure of IgA (2 forms)
as a monomer (2 α chains, 2 κ or λ chains) in serum
as a dimer (contains a J chain and secretory component) in mucosal secretions
Where is dimeric IgA formed?
In plasma cells
-by addition of a J chain
Where is the secretory component added to the dimer?
Added by the epithelial cels during the passage of IgA through glandular epithelial cells
What is the purpose of the secretory component?
Protect the dimer from proteolysis by enzymes found in mucosal secretions
What is the function of IgA in mucosal surfaces?
defends mucosal surfaces from microbial attack by inhibiting pathogen adherence
What occurs to the IgA-antigen complexes ?
trapped in the mucus and eliminated by mechanical action
What is the role of IgA in breastmilk?
Confers mucosal immunity on newborns
who only have IgM at this point, which has a hard time exiting the blood and getting into the tissues
Formation of secretory IgA
- Plasma cell secretes dimeric IgA into the submucosa
- Poly-Ig receptor on inner side of epithelial cells binds the dimeric IgA
- Dimeric IgA taken up into a vesicle
- Addition of secretory component and enzymatic cleavage
- Secretory IgA released from the apical side of the epithelial cells into the lumen
Why do you not find much IgM in mucosal secretions?
The secretory component is not large enough to protect the pentameric molecule from the proteolytic enzymes
Structure of IgE
monomer consisting of 2 (heavy) ε chains and 2 (light) κ or λ chains
What is the general distribution of IgE in serum?
Low generally
High in cases of allergy or parasitic infections
What is the main function/mode of action of IgE
Bind to high affinity Fcε receptors on mast cells and basophils
What causes mast cell and basophil degranulation?
Crosslinking of membrane bound IgE
- 2 IgE molecules bound to the mast cell
- Allergen comes and binds to the Fab regions of both IgE molecules
- degranulation occurs
What is the result of mast cell degranulation?
Acute inflammation and allergic responses
IgE is important in host defense against…?
certain parasitic worms
Structure of IgD
a monomer consisting of 2 δ chains and 2 κ or λ chains
What is the level of IgD present in serum? Why?
Low due to susceptibility to proteolysis
Where is most IgD found?
on the surface of B cells
What is the role of IgD
involved in B cells activation in response to specific antigen
Why are membrane bound immunoglobulin molecules unable to interact with intracellular signalling molecules?
Because they have very short cytoplasmic tails
What 2 elements constitute the antigen binding receptor on B cells
- membrane bound immunoglobulin (mIg)
- Disulphide linked heterodimer Ig- α/Ig-β
* not actually linked covalently but expressed in close proximity
What is the purpose of the Ig- α/Ig-β heterodimer?
the cytoplasmic tails interact with intracellular signalling molecules like tyrosine kinases
What are some of the immunoglobulin superfamily members?
antibody, Ig-α/Ig-β, the T cell receptor, Fc receptors, CD4, major histocompatibility complex molecules, and various cell adhesion molecules
What is the immunoglobulin-fold domain structure?
110 amino acid residues arranged in antiparallel sheets of ß-pleated strands
-held together by intrachain disulphide bonds
What are monoclonal antibodies?
derived from a single B cell clone
What are monoclonal antibodies specific for
All specific for a single epitope on an antigen
how can monoclonal antibodies be generated in vivo?
Fusing an immortal hybridoma cell with plasma cells that can have been exposed to antigen
the hybridomas of each plasma cell and cancer cell will produce a singel antibody
