Lecture 7&8: Antibody Structure & Function Flashcards
what are antibodies aka immunoglobulins?
glycoproteins on the surface of B cells and secreted by plasma B cells
antibodies are present in which bodily fluids?
blood serum, secreted fluids such as saliva and breast milk
immunoglobulins activate which complement pathway?
classical
antibodies act as ___ to ___(increase or decrease) phagocytosis
opsonins; increase
antibodies do what to toxins and viruses (in general terms)
neutralize
antibodies function as ___ receptors for B cells
antigen
the membrane-bound antibodies on B cells have very ___ (long or short) tails; what is the significance of this?
short, so they are not able to interact with intracellular signalling molecules
to act as a B cell receptor, the membrane-bound immunoglobulin is joined to a ______ heterodimer by a disulphide linkage
Ig-a/Ig-B
what is the function of the hetrodimer that is joined to the immunoglobulin of the B cell receptor
they have longer tails that can interact with intracellular signalling molecules like tyrosine kinases
describe the structure of monomer antibodies
2 identical heavy chains and 2 identical light chains held together by disulphide and noncovalent bonds
what are the 5 classes of heavy chains for monomer antibodies?
a, circle with a squiggle on top (could be a delta or sigma), E, y, u)
what are the 2 classes of light chains in monomer antibodies?
K and upside down Y
in the immunoglobulin structure, globular domains of around 110 ____ are formed by intrachain disulphide bonds
amino acids
The V region of heavy and light chains is characterized by a ____ sepquence
variable amino acid
the C regions of heavy and light chains are characterized by a ____ sequence
relatively constant amino acid
Glycosylation of antibodies affects what aspects of their function?
their stability and interactions with other proteins
the antibodies of camels and lamas do not have which antibody chain?
light chain
do all vertebrates have antibodies?
no; things like the hagfish and lampreys have variable lymphocyte receptors (VLR) instead
how many V and C regions does each light chain have?
1C and 1 V
how many V and C regions do heavy chains have?
1V and 3-4 C regions
what chains form the antigen binding site?
the variable region domains at the amino-terminal portion of the heavy and light chains
there are 3 ____ regions within the V region that show even greater AA sequence variability and form the antigen binding site, while ____regions have much less variability
hypervariable; framework
what are the 3 hypervariable regions?
CDR1
CDR2
CDR3
in IgA, IgD, and IgG, what makes up the hinge-region between CH1 and CH2?
proline-rich amino acids
which antibodies do not have a hinge region? What do they have instead?
IgM and IgE; the CH2 domain has hinge-region-like properties
C regions at the carboxyl-terminal portion of heavy chains mediate ____ function
biological effector function
differences in the heavy chain constant regions determines what characteristics of the antibody?
- half life
- distribution
- complemenr-fixing
- Fc receptor binding
the carboxyl terminal domain of membrane-bound and secreted antibody is _____(the same/different) in terms of structure and function
different in both senses
t/f different classes of membrane-bound immunoglobulin molecules are expressed by B cells at different stages of their development
true
the enzymes ___ and ___ can cleave IgG molecules into specific fragments
papain and pepsin
what are the products of papain-mediated digestion of IgG molecules?
2 identical Fab fragments and 1 Fc fragment
what does FAB stand for?
fragment antigen-binding
what does Fc stand for?
fragment cystalizable
what are the products of pepsin-mediated digestion of an IgG molecule?
1 divalent F(ab’)2 fragment and 1 pFc’ fragment
what are the products of pepsin-mediated digestion of an IgG molecule?
1 divalent F(ab’)2 fragment and 1 pFc’ fragment
antibodies are ____; meaning they produce an immune response when injected into other species
immunogenic
isotypic epitopes are located in the ____ regions and they define the ____ and _____ within a species
heavy chain classes/subclasses and light chain types and subtypes
allotypic epitopes are located in ___regions and may vary from ___ to ____
constant; from individual to individual
idiotypic epitopes are located in the ____ regions and are defined by ____ that determine specificity
heavy and light chain variable regions (both hyper and framework); the unique AA sequences
“determinant” is a synonym for ____ and means the same thing
epitope
what is opsonization wrt antibodies (i.e. the reaction)?
interactions of antibodies with Fc receptors on phagocytes to promote phagocytosis
what is antibody-dependent cell-mediated cytotoxicity (ADCC)?
antibody IgG acts as a receptor to enable recognition and killing target cells by NK cells with Fc receptors (CD16)
what is trancytosis?
passage of antibodies across epithelial layers to deliver certain classes (mostly IgA) to mucosal surfaces
the induction of mast cell degranulation is initiated by the ___ receptors of which antibody?
Fc of IgE
what are 5 immunoglobulin effector functions?
- opsonization
- complement activation
- antibody-dependent cell-mediated cytotoxicity (ADCC)
- transcytosis
- mast cell degranulation
what chains make up the IgG structure?
2 identical 50kDa y and 2 identical 25kDa k or upside down y chains
there are ___ (#) subclasses of IgG in humans and each has its own unique biological properties
4
what is the most abundant antibody in serum and extravascular spaces?
IgG
what are the functions of IgG?
- neutralizes viruses/toxins
- opsonizes microbes
- activates classical complement pathway
ADCC
does IgG cross the placenta?
yes
IgM is a monomer when expressed ____ and a pentamer with a J chain when ___
on surface of B cells; secreted by plasma B cells
what chains make up the monomer structure of IgM?
2 u chains and 2 k or upside down y)
how many antigen-binding sites on pentameric IgM are able to physically interact with large antigens?
5-10
what antibody is the first made by newborns and is involved in teh primary immune response?
IgM
IgM is an efficient ___ of particulate antigens and an activator of the ___ complement pathway
agglutinator; classical
IgM is found in ____ (high or low) concentration in the fluid between cells; why?
low; its large size prevents easy movement across blood vessel walls
what allows the IgM pentamer to be transported across epithelial mucosa?
the J chain
where is IgA a monomer? Where is it a dimer?
monomer in the blood and a dimer in mucosal secretions
what chains make IgA?
2 a chains and 2 k or upside down y chains
the dimer form of IgA also contains what 2 extra things?
a J chain and a secretatory component
dimeric IgA is formed by plasma cells by the addition of a ____
J chain
at what stage is the secretatory component added to the dimeric IgA?
during its passage through glandular epithelial cells
what is the function of the secretatory component added to the IgA?
protects IgA from proteolysis
what is the main function of dimeric IgA?
defends mucosal surfaces from microbial attack by inhibiting pathogen adherence
IgA-antigen complexes are trapped in ____ and eliminated by ___
mucus; mechanical action
IgA is present in breast milk, what is the function of this?
provides newborn with mucosal immunity to environmental pathogens
what chains make up the IgE molecule?
2 E chains and 2 k or upside down chains
a ____(small or large) amount of IgE is found in the blood; why?
small; only really present in high amounts when there is an allergic response or parasitic infection
what is the main function of IgE? How does it do this?
mast cell and bosophil degranulation. Binds high affinity FcE receptors on these cells and cross link with other IgE molecules
mast cell degranulation caused by IgE leads to what?
acute inflammation, atopic (Type 1) allergic responses
IgE has an important role in host defence against what types of infections?
parasites, especially certain species of parasitic worms
what chains make up the IgD?
2 delta chains and 2 k or lambda chains
the amount of IgD is very ___(low/high) in the blood, why?
low bc it is very susceptible to proteolysis
where can you find most of the IgD in the body?
on the surface of B cells
what is the function of IgD?
involved in B cell activation in response to specific antigen
molecules that are associated with immune function and have what structure are considered members of the “immunoglobulin superfamily”?
immunoglobulin-fold domain structure (i.e. 110 AA arranged in antiparallel sheets of B pleated strands)
Give 7 examples of members of the Ig superfamily
- Ig-a/Ig-B
- T cell receptor
- Fc receptors
- CD4
- MHC
- antibody
- various cell-adhesion molecules
how is it believed that the Ig superfamily of genes originated?
through mutation and duplication from a single primordial gene encoding a polypeptide of 110 amino acids
where are MABs derived from? What is the implication of this?
from a single B cell clone; this means they are specific for a single epitope rather than for polyclonal AB that are made by many different B cells and so many bind different epitopes
MABs are produced by what types of cells?
immortal hydridoma cells created by fusing an antigen-specific B cell with a cancerous myeloma cell
the hybrid cells used in making MABs are secreted by ____
growth in special tissue culture medium that allows only the hybridoma cells to grow
hybridoma clones that produce a MAB with the desired antigenic specificity are prepared by _____ for use as diagnostic, imaging, and therapeutic agents
propagating them in tissue culture using bioreactors
Sometimes, AB can be found on the surface of mast cells, why is this?
mast cells have a Fc1 receptor
t/f the structure of AB is called a heterotertromer
true
VLRs are similar to AB in struture and function, but not in ____
sequence
VLRs are expressed in what type of vertebrates?
jawless
VLR are made of what? What are they more similar to (compared to AB)?
leucine-rich repeats; more similar to TLRs and NLRs
t/f VLRs can be membrane-bound or secreted
t
what is the purpose of the hinge region?
allows for different size and shaped antigens
Naive B cells have ____ and ___ antibodies on their surface and then once they switch to plasma cells, they secrete ____, ___ antibodies etc.
IgM and IgD; IgA, IgG
where does pepsin cleave the antibody?
right below the disulphide bonds between the 2 heavy chains
where does papain cleave the antibody
right above the disulphide bond of the heavy chains
