JT - Antibody Structure, Function & Engineering Flashcards
What are some examples of anticancer antibody drugs? (3)
- Rituximab (Rituxan)
- Bevacizumab (Avastin)
- Trastuzumab (Herceptin)
What is an example of a human antibody therapeutic?
Adalimumab (Humira)
A fully human antibody that targets TNFα
What are the main components of an antibody? (2)
- 2 light chains and 2 heavy chains
- Linked by covalent (S-S bridges) and non-covalent interactions
What are the functional regions of an antibody? (2)
- Variable (V) region → Responsible for antigen binding
- Constant (C) region → Responsible for effector functions
What is the loop that binds the antigen?
- The Complementarity-Determining Region (CDR)
First point of contact
What are some key features of antibody-based biologics? (7)
- Can exist in various shapes and formats
- Can be adapted to bind to ligands in an appropriate way
- May be used as full antibodies or as fragments
- Monoclonal antibodies - full IgG molecules mostly
- Murine antibodies - derived from mice
- Have the technology to make human antibodies
- Can take elements of an IgG molecule
What is an example of an antibody-based biologic? (3)
Etanercept
- Uses the constant region of an IgG antibody and the TNF receptor
- Used to treat autoimmune inflammatory diseases
What is Certolizumab Pegol, and how does it work? (2)
- A monoclonal antibody fragment
- Uses variable domains to bind specifically to its target
What are the key components of an antibody? (4)
- 2 light chains and 2 heavy chains
- Linked by covalent (S-S bridges) and non-covalent forces
- Variable (V) region – Binds to antigen
- Constant (C) region – Responsible for effector functions
Where is the antigen-binding site located and what is the first point of contact? (2)
- The Variable region (V region)
- Complementarity-Determining Regions (CDRs) - The loop that binds the antigen
What are the steps in antibody gene recombination at the DNA level? (4)
Germline DNA organisation – Separate V (Variable), D (Diversity), J (Joining), and C (Constant) segments
Somatic recombination – DNA segments are cut and pasted
- Light chain: V joins to J
- Heavy chain: D joins to J, then V joins to DJ
What happens at the RNA level? (2)
- Transcription – Rearranged DNA is transcribed into RNA (primary transcript)
- Splicing – Non-coding regions (introns) are removed, and exons are joined (mature mRNA)
What happens at the protein level?
- Translation – mRNA is translated into light and heavy chain polypeptides
- Assembly – Polypeptides fold and form antigen-binding site and a complete antibody
What are the main features of the light chain? (3)
- Variable, Joining, and Constant domains
- Come in 2 types: kappa (κ) (more common) and lambda (λ)
- 70 V, 5κ and 4λ J segments
What are the main features of the heavy chain? (4)
- Variable, Diversity, Joining, and Constant domains
- 40 V, 25 D, and 6 J segments
- Allows for a vast number of different heavy chain variable regions to be generated
- Contributes significantly to the diversity of the antibody repertoire
What are the five main antibody classes?
Class determined by the constant region
- IgA
- IgD
- IgE
- IgG
- IgM
What are some key properties of different Ig classes? (5)
- IgG2 - More stable hinge region than IgG1
- IgG3 - Very long hinge region, altering function
- IgG4 - Can form bispecific antibodies
- IgA - mostly around mucosal surfaces; gut, lungs, tears. Forms dimer
- IgM - First antibody generated in primary B cell response, forms pentamer
What are some functional differences between antibody classes? (5)
IgM, IgG3 – Complement activation
IgG1, IgG3 – Opsonization
IgG2, IgG4 – Placental transfer
IgE – Mast cell & basophil activation
IgA – Secreted onto mucosal surfaces
What happens when an antibody undergoes class switching? (2)
- Heavy chain class changes (e.g., IgM → IgG)
- No change in antigen specificity
What enzyme mediates isotype switching?
Activation-induced cytidine deaminase (AID)
What is somatic hypermutation? (2)
- Mutations in CDR3 regions of variable domains
- Increases antibody affinity for antigens
What is seen in Ig levels and affinities with repeated immunisation? (6)
Increase in antibody concentrations (plasma)
- Due to clonal expansion of specific B cells & memory
Increase in antibody affinities
- Mostly seen with IgG as well as IgA, IgE
- Secreted by switched memory B cells
- “Affinity maturation” due to somatic hypermutation
What is the typical half-life of IgG in humans?
~3 weeks
How does FcRn recycling extend IgG’s half-life? (5)
- Non-specific uptake: IgG enters endothelial cells via pinocytosis, forming an endocytic vesicle (along with other proteins).
- FcRn binding: IgG binds to FcRn receptors inside the vesicle at acidic pH.
- Protection from degradation: FcRn binding prevents lysosomal degradation, prolonging IgG’s lifespan.
- Recycling and release: FcRn-IgG complex is transported to the cell surface and IgG is released at neutral pH, returning to circulation.
- Degradation (alternative pathway): If IgG does not bind FcRn, it is sent to lysosomes for degradation
