Lecture 4-7: Structure & Function of Ig (B Cell Focus) Flashcards
Immunogen
A molecule that is capable of eliciting an immune response by an organism’s immune system Substance which induces a specific immune response e.g. hapten-carrier
Antigen (Ag)
A molecule that is capable of binding to the product of that immune response Target of immune response e.g. hapten
Antigenic Drift
(Slow) change in structure of virus antigen Results in some (otherwise) cross-reactive antibodies failing to bind
True or false: A T cell epitope can be linear or conformational.
False, Always linear (peptide)
True or false: An Ab epitope can be linear or conformation.
True
κ:λ ratio in humans
2:1
IgG
Monomer (bivalent) High affinity
IgM
Has extra C domain (Cμ4) Pentamer (10 valency) - 5 x μ chains High avidity although low affinity
IgA
Dimer (tetravalent) Major Ig secreted into GI and respiratory tracts
IgD
Low abundance Mainly cell-associated Highly expressed on naive B cells Not actively secreted
IgE
Involved in allergic reactions Immediate-type hypersensitivity Involved in response to parasites Binds to mast cells Induces degranulation Has extra C domain (Cε4)
Neutralisation
Binding Ab inhibits/neutralizes pathogenic organism e.g. Ab binds to virus, stops attachment to cell and therefore infection e.g. Ab binds to active site of bacterial toxin Viruses and bacteria bind to cell surface receptors for infection Ab binding to proteins essential for attachment can block infection -neutralisation. Antibody binding to the surface of pathogens may disrupt other processes essential for productive infection e.g. uncoating of viruses
Opsonisation (Indirect targeting mechanism)
Enhanced phagocytosis by coating with Ab Occurs in three steps: 1. Coating pathogen with specific Ab 2. Binding to phagocyte via FcR 3. Phagocytosis of bound particle
Antibody-Dependent Cell-Mediated Cytotoxicity (Indirect/Targeting Mechanism)
Three steps for ADCC: 1. Ab binds to target cell via variable region e.g. Ab binding virus infected cell 2. Ab binds to Natural Killer (NK) cell via FcγRIII receptor (CD16) 3. Cross-linking of FcR triggers NK cell killing
Indirect/Targeting Mechanism - Degranulation
Degranulation (e.g. in eosinophil, mast cells): IgE binds to parasite or allergen IgE binds to FcεR on granulocyte Cross-linking of FcεR triggers degranulation - release of inflammatory mediators e.g. histamine This may not reflect the actual order of events - steps 1 and 2 are probably reversed
IgA Transcytosis
Poly-Ig receptor on epithelial cells which sit proximal to the gut binds the J chain (of IgA and IgM, but more commonly IgA)
The ‘secretory component’ includes the IgA, and a bit of the poly-Ig receptor from where it was cleaved off the gut lumen side of the epithelial cell.
Complement Activation (C’)
Complement is a set of proteins that promote destruction of pathogens and infected cells
Ab binding to C’ component C1q via Fc initiates “classical” complement cascade (only IgG and IgM complexes can bind C1q)
Ag binding can trigger conformational change in pentameric IgM
Upon Ag Binding, IgG can form hexamers
Free Ab does not activate C1q
Functions of C’ (Complement) activation
Recruitment of inflammatory cells and production of inflammatory mediators
Production of opsonins
Deposition of pore-forming proteins
Which Ab isotype is best for neutralisation?
IgG, IgA
Which Ab isotype response is best for opsonisation?
IgG and to a lesser extent IgA
Which Ab isotype response is best for ADCC?
IgG
Which Ab isotype response is best for degranulation?
IgE
Which Ab isotype response is best for C’ activation?
IgM >> IgG, IgA
Allotype
Existence of alleles in Ig loci
e.g. IgG1 will have minor sequence differences between different individuals
Idiotype
Variation in the variable region
These lead to different specifities of Ig
In the Kabat-Wu plot, the region with the largest increase in diversity corresponds to
The region with the largest increase in diversity corresponds exactly to the junction of the V and J segments.
During heavy chain gene rearrangement, which rearrangement process occurs first?
D - J rearrangement
V(D)J recombinase complex
Recombination-activating genes (RAG-1/RAG-2): involved with breaking/joining DNA
Exonucleases: involved in removal of nucleotides and ends generated by RAG’s
Terminal deoxynucleotide transferases (TdT): involved in insertion of random, non-template-encoded nucleotides (N-nucleotides/N-regions)
Sources of Ig diversity
Combinatorial diversity - Combinatorial joining i.e. different combinations of V, D, J for a particular chain and combinatorial association i.e. different combinations of heavy and light chains
Junctional diversity - contributed by random N-region sequences (estimated ~104). Determined by TdT, endonucleases.
Somatic mutation - creates diversity as immune response progresses or “matures”. Mostly found in the secondary response
Progression of the isotype class switching response
IgM/IgD → IgG → IgA → IgE
Terminal deoxynucleotide transferases (TdT)
Involved in insertion of random, non-template-encoded nucleotides
N-nucleotides/N-regions
N-regions contribute to diversity (i.e. almost any sequence is possible in the N-regions)
Activation Induced Cytidine Deaminase (AID)
Induces somatic hypermutation of activated B cells (centroblasts) in germinal centres
Converts cytosine to uridine resulting in the introduction of mutations
