Antibody Structure & Function Flashcards
What are the two forms of antibodies?
Bound to the surface of the B cell - called a B-cell receptor (BCR)
Soluble - outside the B cell
What role does the BCR plays a B cell activation?
Membrane-bound forms help to activate the B cell
Both forms of antibodies are identical with the exception of the ___________________ on the membrane bound BCR form which is removed in order to __________________________
Hydrophobic tail;
Allow passage through the membrane for the soluble form
Soluble forms of antibodies bind __________
Extracellular pathogens and their products (toxins).
T/F. Soluble forms have the SAME specificity as the BCR.
True
When the antibody binds, what do they do to the pathogen?
They don’t harm or kill the pathogen, but deliver it to another cell of the immune system which can remove/kill it
- Antigen is marked for opsonization or ADCC by cells with Fc receptors
5 main isotype of antibodies
IgA, IgD, IgE, IgG, IgM
In which ways are antibodies isotypes similar and different?
- Isotopes share a common structure and overarching function.
- Each isotype has unique properties and specific functions
Antibodies are highly specific for a particular _______ while also being highly diverse in ______________
Epitope; sequence and function
Antibodies are highly diverse in portions of their sequence that provide ____________
Antigen specificity
T/F. Antibodies are highly specific. Each antibody recognizes only a small, specific area on a particular antigen (epitope).
True
Antibodies have conserved sequences that allow them to __________
Carry out common functions
Which part of the antibody molecule determines specificity?
Variable regions
Which part of the antibody molecule determines an anatomic distribution?
Ch- Heavy chains
The __________ regions determine isotype/class. Therefore, different isotype have different __________________.,
Constant regions; biological activities/functions
Heavy chains gross structure
2 H chains are held together by two interchain S-S bonds
Describe the 2 regions of the Heavy chain
Variable (Vh)- different for antibodies produced by cells derived from different clones
Constant (Ch)- same for each antibody within an isotype, but different from one isotype to another.
Gross structure of Light Chains and relation to the H chains
The L chains are bound to the H chains through a single interchain S-S bond
The L chains do not interact with each other
The (heavy/light) chains within an antibody molecule will be (identical/different).
Identical for both heavy and light chains
What are the 2 types of L chains?
Kappa and Lambda
Describe the 2 regions in each L chain
Variable (VL)- different for antibodies produced by cells derived from different clones
Constant (CL)- same for each type (kappa or lambda)
Variable (V) Region.
V region comprises only the first 110 amino acids of the polypeptide chain (remaining sequence is C region).
VL and VH determine antigen specificity
Note: VL=VL; VH=VH; VL NOT = VH
Constant (C) Region
The sequence of the constant part of the chains remains relatively unchanged within each isotype.
Ex.
CH for IgM = CH for IgM
CH for IgM NOT = CH for IgG
CL for kappa = CL for kappa
CL for kappa NOT = CL for lambda
The CH portion of the Heavy chain determines ________.
Function
The CH region of the H chain is further divided into domains (CH#).
IgG, IgA, IgD - have 3 domains CH1-CH3
IgM and IgE - have 4 domains CH1-CH4 (no hinge in these isotypes)
What are the functional regions of the antibody?
Fab- each monomer has a part that binds the epitope on the antigen
Fc- A part that crystallizes under certain conditions
Importance of Fab and F(ab’)2.
Retain the ability to bind antigen.
- they cannot participate in opsonization or ADCC when treated with the enzymes
Importance of Fc
Fc has no ability to bind antigen.
- In the non-digested molecule, this portion serves as the recognition piece for opsonization or ADCC by the Fc receptor on certain cells (ex. Macrophages)
Structure of Fab
2 identical pieces
- comprised of VL, CL + VH, CH1 attached through S-S bonds
Structure of Fc.
Distal portions of both H chains held together by S-S bonds.
Hinge + CH2, CH3 + CH2, CH3
Papain digestion
Cleaves above the hinge region resulting in total of 3 pieces
- 2 identical Fab
- 1 Fc
F(ab’)2
A single fragment that contain both Fab regions, held together by the S-S bonds in the hinge region
VL, CL and VH, CH1, hinge + VL, CL & VH, CH1, hinge
Note- there is no Fc as the remainder of the CH regions below the hinge are degraded into small fragments
Pepsin digestion
Cleaves below the hinge region resulting in 1 piece containing both the Fabs linked together by the S-S bonds in the hinge
- 1 (Fab’)2
Hinge region- structure, function, and located where?
Short stretch of amino acids located between CH1 and CH2
Provides flexibility so Fab’s can bind epitope at various angles & distances apart
Only present in isotypes with 3 CH domains- IgG, IgA, IgD
J Chain-
- Associated with
- structure
- required for
Associated with the polymeric isotypes- IgM and IgA
1 J chain per polymeric Ig molecule
Required for secretion via the plg receptor (plgR)
- crucial to secretory and mucosal immunity results in Slg
Hypervariable regions
Occur in specific locations within the V regions
- are 8-11 amino acids in length
- include the amino acids that actually make contact with the epitope
Where in the antibody structure are the hypervariable regions found?
Occur in specific locations within the V regions
Linear in the sequence, but are adjacent in the 3D folded protein
What is the role/function of the hypervariable region?
Make contact with the epitope
Hypervariable region aka complementarity determining regions (CDRs)
The 6 CDRs within each Fab form the part of the antibody molecule known as the
- antibody binding site
- antibody combining site
What are the framework regions?
The remaining 80 or so amino acids of the V regions
Where in the antibody molecule are the framework regions found?
Interspersed between the hypervariable regions
4 FR within each VL and 4 within each Vh
Summarize Fine Structure for each monomeric antibody.
3 HV regions within each VL and 3 within each VH
6 HV regions per Fab
12 HV per monomeric ab molecule
4 FR within each VL and 4 within each VH
Types of L chains
Kappa
Lambda
Do the different L chains have different roles?
No
No known effector function associated with the CL part of the L chain
An antibody will have (identical or different) L chains
Identical.
Both kappa or both lambda
- polymeric antibodies will have identical monomers
Major types/classes of H chains
γ, α, μ, δ, and ε. They define classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE, respectively
Which H chains classes have subclasses and what are they?
IgG has 4 subclasses: IgG1, IgG2, IgG3, IgG4
IgA has 2 subclasses: IgA1, IgA2
What is isotype switching?
When a B cell changes which isotype it produces
This allows the immune system to fight the same pathogen at all surfaces or areas of the body
Which part of the antibody molecule changes with isotype?
- only the CH portion of the H chain will change
- no changes to CL, VL, or VH
- antigen specificity will remain the same
T/F antigen specificity depends on isotype.
False!
Isotype has NO BEARING on antigen specificity.
Isotype is determined by the C region of the H (CH) & L chains (CL)
VH and VL regions do not change when the isotype changes
What part of the antibody determine antigen specificity?
VL and VH
Which part of the antibody determines function?
Constant region of the heavy chain (CH)
Which part of the antibody molecule determines anatomic distribution?
The CH portion of the antibody molecule
When is IgM first made?
Starts being produced by a fetus at approximately 20 weeks gestation
First isotype to be produced by B cells upon initial activation by T cells (T-dependent response)
Most predominant isotype in response to T-independent antigens
What is a major function of IgM?
Best at activating classical complement
Agglutinate pathogens because of effective cross-linking
Note- too large for routine secretion into tissues
Where is IgM located in the body?
Mostly found in serum, but small amounts can be secreted onto mucosal surfaces (via the pIgR binding to the J chain)- polymer
BCR- monomer
What are some of the structural characteristics of IgM?
Found in polymeric and monomeric forms
Pentameter»_space;»> hexamer
All forms have a CH4 domain and no hinge
Polymer have J chain which allows secretion onto mucosal surfaces
When is IgG first made?
4-6 months after birth
- before this crosses the placenta
What are the functions of IgG?
4 subclasses with varying degrees of functional ability
- Can activate complement
IgG3 > IgG1 > IgG2 - Opsonization
- Neutralize bacteria, toxins & viruses by preventing their entry into host cells
- ADCC via Fc receptors
Where is IgG located in the body?
Most abundant antibody in both tissues and serum
- even though not the most abundant isotype made
What are the different subclasses of IgG? Do they have different functions?
IgG1, IgG2, IgG3, IgG4
Yes!
Numerical designation correlates with their decreasing concentrations in serum
When does IgA first start being made?
Made by an infant approximately 4-6 months after birth
- protective for infants who are breast fed (passed from mother)
What is a major function of IgA?
IgA1- recognizes protein epitope
IgA2- recognizes polysaccharide epitopes
Where is IgA located in the body?
Found mostly on mucosal tissues and in secretions (tears, saliva, sweat, mucus, colostrum)
IgA1 is predominant in serum (2nd most abundant!)
What are the 3 main structural forms of IgA?
Serum IgA (sIgA or just IgA)
Polymeric IgA (pIgA) - this is a dimer and is the form of IgA after it leaves the plasma cells, but before it is secreted via the pIgR
Secretory IgA found in secretions (SIgA)- this is always a dimer and is the form after it is secreted across the epithelial barrier by the pIgR
What is a unique structural feature of dimeric IgA (can also be found on IgM)?
???
What is the secretory component? On which structural type of IgA is it found?
SIgA - always a dimer and is the form after it is secreted across the epithelial barrier by the pIgR
Where is IgE mostly found in the body?
Nearly all IgE is bound to the high affinity Fc receptors on mast cells, basophils, and eosinophils
When is IgE made?
When Fab’s are cross-linked due to antigen binding, the attached cell releases its contents (degranulation)
What is the main “healthy” function of IgE?
IgE binds to parasites, causing degranulation of attached granulocytes to kill the pathogen
- ADCC
What is the main “dysfunctional” response associated with IgE?
IgE binds to allergens, causing degranulation of attached granulocytes
- Allergies (Type I Hypersensitivity)
When is IgD made?
Constitutively expressed on B cells (except when under attack)
?
What is the main function of IgD?
Allows naive, mature follicular B cells to enter lymphoid follicles
If IgD is missing, what might occur with humoral immune responses?
Deficiency of humoral immunity
Which isotypes are found in polymeric form?
IgM and IgA
IgM- pentamer or hexamer
IgA- monomer or dimer
Which isotypes cross the placenta?
Only IgG
Which isotypes activate complement?
IgM and IgG
Which isotypes participate in ADCC?
IgG and IgE
Which isotypes are considered opsonins?
IgG and IgA
Which isotype is the first to be secreted upon activation of the adaptive response?
IgM
Which isotypes have a J chain?
IgM and IgA
Which isotypes have a hinge region?
IgG, IgA, IgD
Which isotype is most abundant in serum?
IgG
Which isotype(s) is/are associated with mucosal surfaces?
IgA and some IgM
Which isotype do we make the most of?
IgA
Which isotype(s) mediate allergic responses?
IgE
Which isotype(s) is/are only found on naive B cells?
IgD
Which isotype(s) provides the majority of our protection to parasitic infection?
IgE
Which isotype(s) has/have a secretory component?
IgA
