Antibody structure/function (notes/class) Flashcards
T/F Abs are effector molecules of B cells but have NO toxic properties and do NOT themselves destroy invading pathogens
True
What is the 1st component of the classical complement cascade? Describe it
opsonization
Bc Abs also serve as ligands for receptors on phagocytic cells, they promote uptake and destruction of pathogen by phagocytes via opsonization
In some cases binding of Abs to a pathogen prevents binding of pathogen to its host ligand which will prevent infection. What is this action?
neutralization
What are globular proteins produced by B cells and primarily from humoral immune response?
Abs
What are the 5 functions of Abs?
- bind to protein or carbohydrate antigens
- interact with host systems (complement) and cells (macs) to promote clearance of infectious agents
- membrane integrated forms serve as main component of BCR
- block spread of infectious agents in blood and help prevent re-infection at mucosal surfaces
- provide protection from rechallenge by infectious agents
All Ab molecules have a common core structure consisting of what?
2 identical light chains
2 identical heavy chains
What forms the attachment of the light chain(LC) to heavy chain(HC)? Describe these attachments
Attachments are formed by intrachain disulfide bonds
1 of 2 LC to each HC
HC are attached to each other
T/F HC and LC are encoded on separate genes
Ig domains fold how? What helps with this?
Ig domains fold independently and interchain disulfide bonds help form the domains
Which Ig domain is the variable region and the constant region?
Variable region is at the amino-terminal Ig domain and is highly variable between different Abs
Constant regions are very conserved
What domains of all Ab molecules have a similar structure?
Constant (C) domain and variable (V) domain
Where do Abs perform their functions? Where do they encounter there that forces them to use a tertiary structure to withstand these?
in extracellular spaces in the presence of infection
encounter variations in pH, [salt], proteolytic enzymes, other destabilizing factors
tertiary sturcture enables them to withstand these factors and maintain functionality
Describe the shape and bonds of the C and V domains of the Ig
both C and V domains are reoughly cylindrical shape and formed by 2 adjacent B-sheet structures covalently linked by a disulfide bond
Describe how Ig folds are formed
adjacent B-sheets form a Beta barrel motif of proteins and those formed in Ab molecules are called from Ig folds
What is the primary difference between the structures of C and V domains?
V domains are larger and have an extra loop of polypeptide chain
flexible loops of V domains form the antigen-binding domains
What forms the antigen-binding site of each “arm” of an Ab molecule?
localized regions of both HC and LC V domains
Sequence variability is confined to 3 distinct regions. What are they?
- hypervariable - HV1, HV2, HV3
- complementarity-determining region - CDR1-3
Low variability between hypervariability are
- Framework regions (FR1-4)
What happens when the HC and LC are paired in an Ab molecule wrt to the HV regions?
HV regions are brought together creating a single HV site at the tip of each “arm” of the Ab molecule
What can the HV site form?
antigen binding site and complementarity determining regions
Depending on the sequences that form the antigen binding site, what can it bind to?
The region of the antigen that is recognized by the Ab is known as what?
the antigenic determinant or epitope
T/F Abs can bind to portions of molecules that vary in shape and physcial properites
True
What are the functions of Abs? When are they most effective?
Fxn: bind to microbes and to facilitate their destruction and removal from body
Most effective: combat infection are those that bind to surface exposed/accessible moecules of pathogen
The portion of any particular antigen that an Ab binds to is called what?
antigenic determinant or epitope
Abs produced in response to an infection usually have a specificity for epitopes composed of what?
either carbohydrates or proteins
Complex macromolecules have multiple epitopes that can be bound by a separate Ab molecule called what?
multivalent antigens
T/F a multivalent antigen can have a single epitope that is repeated many times
false, or it can have a number of distinct epitopes
Abs that have specificity for chem structures other than carbs or proteins can be produced. These are often involved in allergic rxns and autoimmmune diseases. What are the specific for and what are they not involved in?
Abs specific for DNA are NOT involved in defense against infection but are involved in allergic rxns and autoimmune diseases
What forces are needed to bind Abs to antigens?
non covalent forces (electrostatic forces, H bonding, van der Waals forces, hydrophobic interactions)
What are the antigen binding regions of Ab molecules typically rich in? What forces are they usually found in?
typically rich in aromatic AA
parcipitate in many van der Waals and hydrophobic interactions
The HV or CDR of the V Ig domain of LC and HC come together to form a unique shape. What is found in these regions so what action can occur?
AA side chains that reside in these regions to allow the Ab to bind to spcific epitope
The binding stregnth of any Ab to its particular antigenic determinant is known as what?
binding affinity
T/F ALL non-covalent protein:protein interactions are reversible
true
An Ab that has ____ affinity for an antigen stays bound longer than an Ab that has ____affinity.
High affinity binds longer than Ab with Low affinity
Ab can bind to 2 types of antigenic determinants of proteins. What are they and describe them?
- continuous epitopes (linear) => formed by linear stretch of AA
- discontinous epitopes (non-linear) => formed by AA from different parts of polypeptide brought together in folded protein
Ab molecules are subdivided into 5 different isotypes based on HC. What are the isotypes?
Ig A, D, E, G, M
The amino terminal ends of both the HC and LC contain what regions of the polypeptide chains?
Variable (V) or hypervariable regions
Describe the V and C region of the LC
LC have a single V region and single C region
Describe the regions that make up the HC
1 V region
3 C regions for IgG, IgD, IgA
4 C regions for IgM, IgE
Which Ig have a hinge region? What AA is present in the hinge region?
IgG, IgD, IgA
rich in proline residues
Why is the hinge region important?
it allows flexibility of Ab molecule that enables it to more easily bind to antigenic determinants and/or immune components FcR
What are the 2 types of LC?
kappa and lambda
The kappa and lambda types of LC are present in which Ab?
present in all 5 isotypes of Ab
only, 1 of these LC types is present in an individual Ab molecule
What are the subclasses of IgA? IgG?
IgA1 and IgA2
IgG1-4
What Ab can exist as multimeric structures of Ab molecules?
IgM and IgA
What is a distinct feature of the multimeric IgA and IgM? What is the function of this feature
the J chain or joining chain that is disulfide linked to the tail pieces which are extensions at the C-terminus region of IgA and IgM HCs
tail pieces stabilize IgA and IgM multimers
What is produced by epithelial cells that line the luminal surface of mucosal tissues? What is the function of this?
the secretory component of IgA
it combines with IgA produced locally by lymphocytes and transports IgA through epithelial cells into secretions
Which Ig comprises serum Ig and has a half life of 6 days?
IgA
IgA occurs as what type of structures?
Where is IgA found?
monomeric, dimeric, trimeric structures
IgA is foundin colostrum/breast milk, intestinal and respiratory secretions, saliva, tears, other secretions
Production of IgA requires what?
What occurs in IgA-deficient individuals?
requires T cell help
IgA deficients experience increased incidence of respiratory infections
Serum IgA is found in what?
Secretory or exocrine IgA is found in what?
serum IgA found in bodily secretions
secretory IgA have mucosal immune stimulation and required for production of sIgA
What is a heavier Ig that is found in low concentration of serum?
IgD
What is primarily a membrane Ig? What Ig does it carry out its function with and what is the function?
IgD
IgD serves with IgM as an antigen receptor on early B cell membrane to help initiate Ab responses by activating B cell growth
What Ig lasts only a short time and found in low concentration of serum?
IgE
IgE is important for protection against what? What is it also responsible for?
protection against parasitic infection
responsible for anaphylactic hypersensitivity
How does IgE protect against parasites?
most IgE is bound to Fc receptors on mast cells where it serves as a receptor for allergens and parasite antigens
If sufficient antigen binds to IgE on mast cells, what occurs?
- mast cells degranulate
- release histamine, prostaglandins, platelet-activating factor and cytokines
What Ig is most prevalent in adults along with the longest half life?
IgG
Which Ig requires T cell help and has a high binding capacity for antigens?
IgG
Which Ig can cross the placenta?
IgG
What is the principle Ab isotype in memory immune responses?
IgG
What are the actions of IgG?
- fixes complement (requires multiple IgG)
- stimulates chemotaxis
- acts as an opsonin that facilitates phagocytosis
What is a pentameric molecule that does NOT require T cell help for production? What stabilizes this structure?
IgM
stabilized by a J chain
What is the most efficient Ig for fixing or binding to complement?
IgM
T/F a single pentmer of IgM can activate the classical complement pathway
true
By IgM activating the classical complement pathway, what does this promote?
phagocytosis and bacteriolysis through complement fixing activity
T/F IgM cannot cross efficiently into tissue
true due to its large size
What Ig is found on the surface of all nairve B cells that serves as a receptor for antigen?
monomeric IgM along with IgD
What Ig is important for immunity to polysaccharide antigens?
IgM
Ab molecules have 2 major structural regions that mediate the 2 primary fxns of Ab molecules. Name them
- variable region/antigen combining sites
- stem portion of Ab molecules are the Fc portion
Describe the variable region/antigen-combining sites along with the idotype
regions of Ab which interact with epitope of antigen and idiotype refers to AA sequences w/in variable region of Ab molecule and sequence forms mirror-image of epitope that the Ab recognizes
Describe the Fc region
interacts with complemetn components with macs and NK cells to promote clearance of antigen and activation of subsequent immune responses
The hinge region is sensitive to proteolytic cleavage. What Ig contain these?
IgG and IgA
Describe the type of proteolyic cleavages of the hinge region of IgG and IgA
papain cleavage yields a single Fc fragment and 2 Fab fragments=>each Fab fragment contains a single antigen-binding site
pepsin cleavage produces a Fab2 fragment that contains 2 antigen binding domains and a single pFc fragment
The role of Abs in effector mechanisms depends on interaction with what?
host cells and complement proteins
Describe how the Fc regions of various isotypes of Ab can be recognized by specialized receptors (Fc receptors) on immune effector cells
- Fc portion of IgG1 and IgG3 Ab are recognized by Fc receptors on macs and neutrophils
- Fc portion of IgE binds to Fc receptors on mast cells, basophils and activated eosinophils
What happens when the Fc portion of IgG1 and IgG3 are recognized?
the cells can bind to and engulf opsonized pathogens
What occurs when the Fc portion of IgE binds together?
respond by releasing inflammatory mediators
What Ig are involved when the Fc portions of Ab:antigen complexes bind to complement and initiate the complement cascade?
IgG1-3, IgM
The Fc portion can facilitate delivery of Ab to sites they would not reach without active transport. What are these sites and what Ig are associated?
IgA => mucous, tears, milk
IgG1-3 => fetal blood circulation
Abs are produced in large quantities by what?
fully differentiated B cells called plasma cells
What is the antigen-binding molecule of B cells?
Igs
Ab molecules are identical to the Ig component of what?
BCR
T/F Each Ab produced by a single B cell has the exact same specificity for antigen
true
During clonal selection, each B cell displays many copies of what on its surface?
the same BCR
Why are the Abs produced by a single B cell the same?
They are made from the same light chain and same heavy chain
Light chains of Abs consist of what type of chains
a single heavy chain and a single light chain
Any AA change in the constant region, will have what? is this normal?
It will cause a change in the B barrel region so is not normal
Where are the antigen binding sites?
hypervariable sites on both the heavy and light chains
What does the binding within the chains of the Ig?
disulfide bonds
What is the importance of the Ig domains?
Fc receptor and complement binding sites
Which terminus contains the V domain?
N terminus
Why are the B barrel sheets of the V domain mostly them same bw all Abs?
changing the AA sequence in a large way may affect the way the Ab is secreted as well as its binding and structure ability
What regions of the Ab give it is antigen specificity?
the loop region that is hypervariable only in the V domain
Each monomeric Ab is composed of what?
2 identical light chains and 2 identical heavy chains
What is any molecule that is bound by an Ab or TCR? What part of this molecule does the binding?
antigen
antigen determinant or epitope
Ab binding is what type? Describe affinity
non covalent and reversible
higher affinity then bound tighter
What can Abs bind to?
- native or denatured proteins or peptides
- carbohydrates, nucleic acids, small molecules and chemicals
- soluble or particulate antigens
What type of forces are involved in Ab binding?
- electrostatic forces
- hydrophobic interactions
- van der Waals forces
- H bonding
What types of epitopes can Ab bind to? Which involves the tertiary structure? Which will not be recognized if denatured?
linear epitope
discontinuous epitope (must be in tertiary structure and will not be recognized if denatured)
T/F pathogens are typically multivalent antigens
true
What serves as a bridge bw the acquired and innate immune system?
secreted Abs
What are the roles of secreted Abs?
opsonization
- mark for NK cells for antibody dependent cell mediated cytotoxicitiy) ADCC
- activate complement pathway
**neutralization **
What serves as specific antigen receptors for B cells?
membrane bound Abs
What Ig has different hinge regions within its structure?
IgG and its subclasses have variable hinge regions
Which Ab isotype protects newborns from common infections?
IgG because it can cross placenta
but also IgA because it is secreted in breast milk
Which Ig are most important in neutralization?
IgG and IgA
Which Ig are most important for opsonization?
IgG
Which is most imporant for sensitization for killing by NK cells?
IgG
Which is most important for the sensitization of mast cells?
IgE
Describe the level of importance for activation of complement system
IgM > IgG > IgA
Which Ab can be transported across the epithelium?
IgA
(rarely IgM)
Which Ig is transported across the placenta?
IgG
Describe the level of importance for diffusion into extravascular sites
IgG > IgA > IgE > IgM
Describe the level of concentration in the serum
IgG > IgA > IgM > IgD > IgE
Describe how B cells can be signaled to produce Ab of a different isotype
- only constant region of HC is altered
- differences in constant chains of Ab isotypes allows max. versatility of Ab-mediated immune responses
Describe the 2 major structural regions of Abs and their related function
Fc region => binds to complement proteins and Fc receptors
Fab => contains antigen binding sites in the V region
