B cells and antibodies Flashcards
a) what is an antibody
b) 5 main heavy chain isotypes (which is made first)
c) light chain types
a) a soluble version of the BCR
b) IgM, IgE, IgG, IgA, IgD. IgM is made first
c) κ or λ
a) where is variation concentrated
b) where is the most variable
a) In the CDR regions
b) CDR3
Process of generating a variable heavy chain domain
Somatic recombination and VDJ joining.
Occurs on chromosome 14. There are 3 important segments involved - the variable (V), the diversity (D), and the joining (J). There are 46 possible V, 23 possible D and 6 possible J (hence huge number of possibilities). Firstly, there is D-J joining, then V-DJ joining (see image). Splicing is used to generate a mature transcript. Rearrangement can occur on both chromosomes. If there is success at the first attempt at rearrangement, then rearrangement on the other chromosome is halted - allelic exclusion (we only want 1 allele from chromosome 14, otherwise it could cause autoimmune problems)
Recombination takes place on DNA and uses the enzymes RAG1 and RAG2 to rearrange gene segments - this is NOT splicing!
VDJ joining occurs on DNA in a precise order. It uses RAG recombinases that recognise conserved heptamer and nonamer sequences. Results in permanent alterations to DNA (intervening DNA is lost)
a) light chain rearrangement
b) result of combinational diversity
a) Occurs on chromosome 2. There are just V and J segments. 40 V and 5 J.
b) generates a large number of different molecules - up to 2 million different ones!
Outline the four main processes of generating antibody diversity
1) different heavy and light chain combinations
2) selection of different V, D and J segments
3) variable addition and loss of nucleotides at VDJ junctions (junctional diversity). Addition by TdT. Addition due to recombination mechanism. Deletion of nucleotides
4) somatic hypermutation. Point mutations introduced into heavy and light chain variable regions. Activation induced deaminase (AID) causes deamination of cytosine to uracil. Antibodies with increased affinity for antigen are then selected by affinity maturation (approximately 1 aa change per cell division)
Affinity maturation of B cells
In lymph node. Somatic hypermutation generates 1 aa change in the BCR per generation. The B cells compete for an antigen displayed on a follicular dendritic cell or T cell. The one that outcompetes the others is the one with the highest affinity for that particular antigen. This repeats for several generations.
Isotype switching
Changes a B cell’s production of antibody from one class to another. The constant region of the antibody’s heavy chain is changes, but rhe variable region remains the same (so no change in antibody specificity). Allows the production of different antibody isotypes that have defined roles in the immune system.
Occurs by class switch recombination. ds breaks are generated in DNA at conserved switch (S) regions that are upstream from the gene segments coding for the constant region of the heavy chain. The DNA is broken at two selected S-regions by several enzymes (including AID and AP endonucleases). The DNA between S regions is deleted, removing unwanted μ or δ heavy chain constant region exons, and allowing the substitution of a γ, α or ε constant region gene segment. The free ends are then rejoined by NHEJ.
Difference between soluble and membrane bound antibodies
Soluble has the S domain (antibody). Membrane bound has the TM (transmembrane) domain (BCR)
Specialised functions of antybody isotypes
Recognition of antibodies by Fc receptors
a) FcγRI
b) FCγRIIa
c) FCγRIIb
d) FCγRIIIa
e) FCγRIIIb
a) High affinity for IgG. Found on macrophages and activated PMNs. Induces phagocytosis
b) Low affinity for IgG. Found on macrophages and neutrophils. Induces phagocytosis
c) Low affinity for IgG. Found on B lymphocytes. Induces antibody regulation
d) Low affinity for IgG. Found on NK cells and macrophages. Induces ADCC
e) Low affinity for IgG. Found on neutrophils. Induces phagocytosis
Antibody dependent cell-mediated cytotoxicity (ADCC)
Some larger parasites (eg worms) are too big to ingest. Therefore, eosinophils can release toxic granules to kill the parasite
Mobilisation of inflammatory mediators
When pathogens cross epithelial barriers, one line of defence is via the mast cell. (see image)
a) antibody transport
b) antibody feedback
a) Using a Poly-Ig receptor and a J chain
b) If an antibody/antigen complex occurs (ie many antibodies created), will switch off B cells
Affinity vs avidity
Affinity is the strength of the binding of one antigen to one antigen-binding site.
Avidity is the overal strength of binding (ie more antigen-binding sites = higher avidity)
