Antibody Structure and Function Flashcards
Describe an antibody and briefly what it does
Antibodies are Y-shaped molecules expressed by immune cells (B cells) and act as the effector arm of humeral immunity
They can work by preventing bacteria from entering cells, by binding to their docking sites, or they can neutralise them via toxins, etc. They also bind foreign (or sometimes self) antigen to recognise pathogens
Describe the structure of an antibody
- An antibody is made up of 2 heavy and 2 light chains. The types of heavy chains are μ, δ, γ, α or ε chains; these can be divided into further classes. Heavy chain has 3 constant domains and one variable domain each
- The types of light chains are κ or λ chains. Each chain has one constant and one variable domain.
- 2 regions: Fab region (top of Y) - determines specificity, Fc region (bottom of Y) determines function
- The chains are stabilised by S-S bonds and can be glycosylated (add sugar molecules) to modulate function - mostly in the Fc (bottom region)
- The variable region of the antibody binds to the antigen, CH1 support.
- The constant region executes effector functions (such as activating complement, binding phagocytes, etc.).
Antibodies are part of the Immunoglobulin super family. Name other members of the family and what makes the family unique?
All members of the family express they all possess the immunoglobulin domain or fold. The domain is around ~110 amino acids long with a loop around 60-70 aa stabilised by non-covelanet interactions and a S-S bridge. This allows for a stable structure and many functional variants
Members of the IgSF:
- cell surface antigen receptors (Surface antibodies),
- co-receptors
- co-stimulatory molecules,
- antigen presentation molecules (TCR, MHC),
- lymphocytes
- cell adhesion molecules,
- certain cytokine receptors and intracellular muscle proteins.
What is the function of the Fab region?
The Fab region is responsible for the antigen binding site at the variable region on both the light and heavy chain. It determines specificity, affinity and avidity of the interaction with the antigen
What is the function of the Fc region?
The Fc region confirms the type and function of the antibody. It is recognised by the Fc Receptor (FcR) on the cell so when the antibody binds via this receptor, the Fc region tells the cell to do something.
Describe the two forms of antibodies.
Antibodies have two forms: the final form will be the secreted form. During its development, it is anchored to the plasma membranes of B cells. At that stage, it is called a B cell receptor.
Throughout its development, it will incrementally secrete antibodies until the last stage of development, where it becomes a plasma cell and exclusively secretes antibodies.
The antibodies are secreted as monomers, but when they come together they can form polymeric structures.
What are the different classes and subclasses of antibody
5 classes & 9 subclasses of antibody (in humans) determined by: Gamma x 4, Meuw x1, Alpha x2 , Delta x1 and Epsilon x1
IgG x 4 - IgG1, IgG2, IgG3 and IgG4
- G1 has highest abundancy in serum
- G4 has the lowest abundancy in serum
IgM x1 - biggest Ig (pentamer)
IgA x 2 - IgA1 and IgA2,
- both heavily glycosylated
- IgA1 has higher concentrations in serum
- IgA2 has higher secretion concentration
- secretory IgA (sIgA) -> formed as a dimer of 2 IgA monomers
IgD x1
- hardly any in the serum, mostly expressed on cells
IgE x1
- the least abundant Ig
Describe the features and functions of IgG
- Monomeric with 3 CH domains, with flexibility in the hinge region
- It has a higher affinity than IgM
- It is the major circulating Ig (70-75% of serum antibodies) and is vital for the secondary response
- Can cross placenta and protect foetus > neonate will have maternal Ab to protect them -
- Activates complement (Classical) and acts as an Opsonin
Describe the features and functions of IgM
- IgM is the first class to be produced from B cells on the cell surface in the primary response in association with IgA and IgB chains stimulation (transducing molecules - also CD8/4)
- allows signal from BCR (Ab) to trigger the cell
- 4 Ch chains
- It constitutes ~10% of serum Ig
- It is in a Pentamer – star-shape effect with up to 10 binding sites > capable of binding antigen depends on antigen
- The pentamer is stabilised by J-chain
- Affinity of each binding site doesn’t have to be great so has low affinity but high avidity
- Activates complement (Classical)
Describe the features and functions of IgA
- IgA forms a range of polymers but in humans, it is mostly monomers or dimer (sIgA)
- Dimer is formed by the 1 J chain per polymer
- 15-20% serum Ig in the body
- IgA has 2 subclasses: IgA1 in serum and IgA2 in mucous - Found in tears, milk (Transfer IgA through breast feeding) , saliva, sweat etc
- Thought to be really important in the protection of external surfaces- first line of defence (other than the skin)
- Can get localised mucosal response – mucosal associated lymphoid tissue (immune system) - localised response
- Does not activate complement (by classical pathway)
How does serum IgA become sIgA and what is the function of sIgA?
- serum IgA dimers are transported across the mucosal epithelium with a poly-Ig receptor on the basolateral surface
- it is then endocytosed and forms a vesicle inside the lumen of the epithelial cells which adds a secretory component to protect against degradation by proteolysis in the gut
- Vesicles then fuse with the luminal surface (inner surface), the sIgA dimer is released and the receptor is cleaved by proteolysis
- Function – primarily acts by blocking epithelial receptor by binding their ligands on pathogens
o It’s a poor opsonin and complement activator
o Simply binding the pathogen might not be enough to contain it
Describe the features and functions of IgD
- IgD is usually a monomer, like IgG but with a delta gene encoded heavy chain with 3 CH domains
- Very little of IgD is serum - <1% serum Ig
- It has specific antigen binding in FAB region but NO effector functions – no receptors for IgD
- It also acts as an antigen specific receptor on B cells together with IgM monomer so exhibits the same diversity of antigen specificity - but could be due to how the body selects the appropriate cells to get out of the bone marrow
- Sensitive to proteolytic degradation and heat
- Involved in antigen triggered B-cell differentiation
Describe the features and function of IgE
- IgE is a monomeric Ig molecule with the heavy chain coded by epsilon-gene.
- Heavy chain has 4 CH domains
- ~50% of IgE is in the serum but is still a very small trace which is elevated in allergic or heavy parasitic infection
- Majority bound to mast cells and basophils through high affinity FceR1 (Epsilon receptor)
Function - key to allergic response and has an important role in parasitic infections, with mast cells being very effective at killing multicellular organisms
What is a BCR made of and what are its functions?
- BCR is composed mainly of either surface bound IgD or IgM Abs (can be IgA, IgG or IgE) which are associated with Igalpha (CD79a) and Igbeta (CD79b) heterodimers (signal transduction) – similar to TCR and CD3
- Both IgD and IgM have the same specificity on each individual cells
- BCRs have a very small intracellular section – why Ig alpha and Ig beta are necessary
• BCRs also have ITAM (Immunoreceptor Tyrosine-based Activation Motifs) molecules, on the cytoplasmic chain of the heterodimer to help trigger the cell
• When an antigen/ligand (?) binds to the BCR, the ITAM phosphorylates
o Leads to downstream cascade of events resulting in differentiation into plasma cell and antibody production
Outcome of BCR ligation:
- Clonal expansion: one B cell when activated becomes a clone of cells each with exactly the same BCR specificity – most are transformed into plasma cells - make antibodies
- A small proportion of cells remain as long-lived memory cells
- The majority of cells become effector cells (plasma cells – protein producing cells with lots of endoplasmic reticulum and mitochondria ) which produce antibody, lots of it!
- Plasma cells have a limited life-span and apoptose after a few days – done their job and produced lots of antibodies
How do scientists produce monoclonal antibodies and give some clinical examples?
- Take an known antigen and inject it into a mouse
- Mouse develops an immune response against the antigen
- You then take the B cells out of the mouse and fuse the B cells with myeloma cell (constantly growing)
- Get a hybridoma, an antibody producing cell that won’t die and keeps producing Abs = Monoclonal antibody
Examples:
- Infliximab -> anti-inflammatory drug against TNF-a used for autoimmune diseases like Rheumatoid arthritis or Crohns disease. It is chimeric - different parts of Ab from different species
- Gemtuzmab - anti-cancer drug which targets myeloid cell surface antigen CD33 on leukaemia cells -> used to treat relapsed AML. Its a humanised Ab - human but changes
