Antibodies Flashcards
What are the main structural components of an antibody, and what are their functions?
2 heavy chains, 2 light chains
Antigen binding fragment (Fab)
○ Variable and constant regions
○ Complementarity determining regions
Constant fragment (Fc)
What are complementarity-determining regions (CDRs), and why are they important for antigen specificity?
Parts that bind to antigen
Undergo somatic recombination
What is somatic recombination, and how does it contribute to antibody diversity?
Rearrangement of gene fragments encoding the hypervariable (HV) regions of the immunoglobulin AKA the complementarity determining regions (CDR)
Many individual copies of each segment - different combinations can lead to diversity
What gene segments are present in heavy and light chains?
Light chain - V J C
Heavy chain - V D J C
What role do RAG1 and RAG2 play in somatic recombination?
RAG 1 and 2 bind to the RSS (recombination signal sequences) and cleave the DNA
RAG 1 and 2 are lymphocyte specific and only expressed during the developmental stage of antigen receptor assembly
What is the process of non-homologous end joining (NHEJ) and how does it increase antibody diversity?
RAG1 and RAG2 bind the RSSs and cleave the DNA -> 2 hairpins
Ku70:Ku80 complex binds the hairpin ends then DNA-PK:Atermis complex joins the complex
Artemis cleaves the hairpin at random points (more diversity)
The nucleotides from one strand can hop over to the other (more diversity)
Artemis has endonucleotidase activity - might remove P nucleotides (more diversity)
Terminal deoxynucleotidyl transferase (TdT) randomly adds up to 20 nucleotodes (more diversity)
DNA repair enzymes remove any unpaired Ns
DNA ligase IV:XRCC4 ligates ends
What is somatic hypermutation, and when does it occur?
process where B cells introduce mutations in the variable regions of their antibody genes
enabling the production of antibodies with higher affinity for antigens.
It occurs in germinal centres during immune responses.
What is class-switching, and how is it regulated?
DNA rearrangement of a constant domain generates diversity in the constant region
Isotype switch determined by specific cytokine combinations
Dependent on T-cell help and CD40-CD40L interaction
Required to induce expression of the enzyme involved in DNA rearrangement required for class switching = AID enzyme
Therefore requires cytokines and CD40-CD40L interactions
How do antibodies neutralize pathogens through Fab-mediated mechanisms?
Antigen recognition
Blocking binding/adhesion of pathogen to host (IgA, IgG, IgM)
What are examples of Fab-mediated blocking of bacterial and viral adhesion?
Toxins
○ Bacteria release toxins -> antibodies bind toxins -> prevent binding to receptor therefore neutralise toxins
Adhesin blocking
○ Adhesins allow bacteria to bind to surfaces of cells and become internalised
○ Ab binding to adhesin prevents internalisation of bacteria
Viral spike proteins
○ Abs bind viral spike proteins -> prevent binding to receptor -> prevents internalisation of the virus -> neutralise the virus
How do antibodies neutralize pathogens through Fab-mediated mechanisms
IgM, IgG interacts with C1q -> complement activation
IgG interacts with FcyR on NK cells -> antibody dependent cellular cytotoxicity (ADCC)
IgG interacts with FceRs on mast cells -> histamine release
How do Fc regions of antibodies mediate complement activation?
Conformational change of IgM upon binding to target (planar -> staple)
Close association of Ag for IgG binding
Opsonisation with C3b enhances phagocytosis via interaction with complement receptors
Describe how IgG Fc regions interact with Fcγ receptors on macrophages, neutrophils, and NK cells.
Ab binds to antigen
Fc receptors recognise Fc tail of Ab
Cross-linking of Fcy receptors -> activation and phagocytosis
What is antibody-dependent cellular cytotoxicity (ADCC), and which cells are involved?
IgG can interact with FcyRIIIa/CD16a on NK cells
NK cells kill target cell by inducing apoptosis referred to as antibody dependent cellular cytotoxicity
What are the different situations in which Fc mediated functions are relevant for bacterial or viral infections?
Fc mediated functions relevant for bacterial/fungal rather than viral
Ab binds bacteria -> complement activation/FcR mediated phagocytosis (macrophages and neutrophils)
The only time Fc is relevant for viral infection is when it mediated ADCC by NK cells
How can autoantibodies contribute to autoimmune diseases?
When antibodies recognise self-antigens therefore antibody functions are targeted at our own cells
○ Cell/organ bound antigen -> local/organ specific pathology
○ Soluble antigen -> systemic pathology
AI diseases happen when B-cells sneak through the mechanisms of control we have in place
What is the role of Abs in the development of pernicious anaemia (AI disease)?
Autoantibodies against intrinsic factor (IF) prevent binding to VitB12 -> prevents uptake
○ vitB12 is essential for haematopoesis and IF facilitates its uptake into blood
Auto-antibodies against parietal cells (which produce IF) -> damage to cells -> no IF -> prevents uptake of VitB12
Treat with intramuscular VitB12 injections (bypasses GI tract)
What is the role of Abs in the development of Grave’s disease (AI disease)?
Autoantibodies stimulate the thyroid hormones
These act on the pituitary gland to shut down the production of TSH
But thyroid keeps pumping out hormones High T3 (triiodothyronine) and high T4 (thyroxine), low TSH
Hyperthyroidism
How can auntoimmune Abs be passed on to a foetus?
IgG can be transported through placenta via neonatal Fc rfeceptor (FcRn)
If a mother has an autoimmune disease, e.g. Grave’s disease, the child may suffer the same disease
Plasma exchange, removing the autoantibodies, cures the disease
What is the role of Abs in the development of Autoimmune encephalopathy (AI disease)?
Inflammation in brain
Anti-NMDA receptor antibodies cause internalisation of NMDAR
Glutamate is release from pre-synaptic terminal but NMDARs (ion channels) aren’t there so post-synaptic neuron isn’t able to generate an action potential
What is the role of Abs in the development of myasthenia gravis (AI disease)?
Muscle weakness
Anti-AChR antibodies cause MG by multiple mechanisms:
○ Bind and directly block AChR signalling
○ Stimulate AChR internalisation
○ Complement activation - MAC formation -> damage of smooth muscle membrane
Treatment:
○ AChE inhibitors -> boost cholinergic signalling
○ Elculizumab = anti-C5 mAb prevents MAC pore formation and C5a mediated effects
What is the role of Abs in the development of AI haemolytic anaemia (AI disease)?
Autoantibodies against RBCs
Ag sparsely distributed
○ Ab interacts with FcRs on special phagocytic cells from ‘fixed mononuclear phagocytic system’ in the spleen and liver
○ Phagocytosis removing Ig-bound RBCs from circulation
Ag densely distributed
○ Ab activates complement -> lysis -> haemoglobinuria
Usually caused by IgGs
Treatment - removal of spleen (spleen involved in B-cell development and where cells that remove cells from the circulation exist)
What is the role of Abs in the development of immune thrombopcytopenia (ITP) (AI disease)?
Platelets are essential for coagulation
Autoantibodies against platelets -> phagocytosis of platelets by macrophages in spleen -> increased bruising
Treatment:
○ Corticosteroids: suppress inflammatory genes and induce expression of anti-inflammatory genes, inhibit T-cells giving help to B-cells -> reduced Ab production
○ Intravenous immunoglobulin (IVIG)
○ Therapeutic monoclonal antibodies: B-cells are producing faulty antibodies - use Rituximab = anti B-cell therapy
What is the role of Abs in the development of Goodpasture’s syndrome (AI disease)?
Auto-antibodies bind to glomerular basement membrane of kidney and alveoli of lung
