Lecture 23. Antibodies and gene rearrangement Flashcards
(31 cards)
what makes the immunoglobulin and T-cell loci unique
ability to rearrange themselves leading to a diverse range of immune responses(antibodies)
adaptive immunity
-has memory
-affinity of B-cells towards antigen increases with time and persistence of antigen’
-born with a massive repertoire of B and T lymphocytes
Each lymphocyte represents a different antigen specificity
Where does B-cell diversity come from
rearrangement of the genes coding for the antigen receptors( different AA composition of receptors)
Where does maturation of B-cells happen
lymph nodes
generation of higher affinity antibodies
Origins of adaptive immunity
-first observes in jawless fish(agnathans) that evolved ~500 million years ago
-A transposon inserted into a primordial receptor gene 500 million yrs ago. Transposase moved away from the receptor gene( can rearrange other genes without affecting its own position in the genome)
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Origins of adaptive immunity
-first observes in jawless fish(agnathans) that evolved ~500 million years ago
-A transposon inserted into a primordial receptor gene 500 million yrs ago. Transposase moved away from the receptor gene( can rearrange other genes without affecting its own position in the genome)
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transposon
Transposons are repetitive DNA sequences that have the capability to move (transpose) from one location to another in genome.
RAG1 and RAG2 ( recombination activation gene)
transposases
The RAG1 and RAG2 proteins initiate V(D)J recombination by introducing double-strand breaks at the border between a recombination signal sequence (RSS) and a coding segment.
Recognition sequences
Base pair sequences found at the ends of any gene segments that rearranges
RS- are the substrate for RAG1 and RAG2 directed recombination.
*RAG1, RAG2 and RS are identical in all species that possess adaptive immunity.
The difference between transposon and loci that undergo gene recombination in mammalians:
RAGS no longer exist on the bits of DNA that move. It is the big event that allowed the phenomenon of gene recombination:
The enzymes RAGs operate on DNA segments without having to reposition themselves.
The difference between transposon and loci that undergo gene recombination in mammalians:
RAGS no longer exist on the bits of DNA that move. It is the big event that allowed the phenomenon of gene recombination:
The enzymes RAGs operate on DNA segments without having to reposition themselves.
Basis of gene recombination:
Genes encode enzymes–> enzymes find Recognition Sequences, cut and reposition DNA segments
Immunoglobulin protein fold
Antibodies are formed from repeated protein units - Ig domains
Ig protein domain fold is called beta-barrel ~110 AA
Two anti-parallel β-pleated sheets joined in the middle
by a disulphide bond.-very stable
The loops at the ends of the strands(antigen-binding surface) are not
constrained so they can vary their amino acid
sequences without affecting the stability of the fold
Antibody protein structure
• The antibody molecule consists of 4 protein chains that are all made up of
repeating Ig domains. There are 2 domains in Light (L) chains and 4-5
domains in Heavy (H) chain.
• 1 H chain is disulfide linked to 1 L chain. The two H chains are disulphide
linked
The “Y” shaped antibody has two flexible arms and the antigen-binding sites
are located at the tip of the two arms. Formed from the N terminal domains
of the L and H chains.
• The effector region is invariant and is bound by Fc receptors and
complement component C1.
what bonds are between the antibody H and L chains
disulphide
five Ig classes
-depends on which H chain is used. Coded for in the H gene locus
IgM
default antibody produced by naive B-cells
- comes in a membrane-bound monomer and soluble pentamer forms
- membrane form is a B-cell antigen receptor(BCR)
- Soluble has 10 antigen binding sites
- strong avidity binding, low affinity
- Good at fixing complement with 5 Fc regions that bind complement component C1.
- matures into IgG
IgG
produced from IgM in the process of maturation
can cross the placenta, pass on to infants from mothers
IgE
least abundant
one of the most medically important, responsible for atopic allergy
Affinity vs avidity
Affinity- When the sum of the attractive molecular forces
at two surfaces exceeds the repulsive forces, there is affinity.
Can be measured
*the higher the affinity the stronger the antibody binds the antigen and the more reactive the B-cell is.
Avidity- “ sum of affinities” Avidity results from multiple
affinity contacts. Like Velcro, the strength of binding can
be orders of magnitude higher than the individual
affinities.
*the way the immune system is able not to react to your own cells but react to foreign cells due to the difference in avidity binding
Complementarity
An antibody can form complementarity to
virtually anything because the potential amino
acid diversity at the antigen-binding site is vast!
• Affinity arises when the sum of the attractive
forces exceed the sum of repulsive molecular
forces.
hydrophobic interactions- 2 hydrophobic surfaces bind together
affinity maturation
the process whereby the immune system generates antibodies of higher affinities during a response to antigen
an antibody can the bound to an antigen irreversibly for the rest of its lifetime(until removed from the blood)
affinity maturation
the process whereby the immune system generates antibodies of higher affinities during a response to antigen
an antibody can the bound to an antigen irreversibly for the rest of its lifetime(until removed from the blood)
How can so many different antibody molecules (>1011)
be produced from so few genes (<30,000 in the human
genome)?
Amino acid variation is found in 3 discrete regions called
Complemetarity Determining Regions (CDR). These
are the 3 loops that connect the strands in the 1st domains
of the H and L chains.
3 loops from VH and VL juxtapose in the folded protein to
form a roughly rectangular surface of ~800-1000 Å2
