Lecture 23: Antibodies and gene rearrangement Flashcards
What are the features of adaptive immunity?
- has memory - secondary response is stronger and faster than primary response
- affinity of B cells towards antigen increase with time and persistence of antigen
- humans are born with a huge range of B and T lymphocytes - each represents a different antigen specificity randomly produced by rearrangement of the genes for the antigen receptors
What is a transposon?
A DNA sequence which can move its position in the genome
This gene was inserted into a receptor gene and transposase enzymes were extracted
What is RAG?
Recombination activation genes are enzymes in human genome that allow movement rearrangement of genes
There are two types RAG1 and RAG2
What are recognition sequences?
RS - are base pair sequences found at the end of genes which rearrange. Recognition sequences are where recombination activation genes (RAGs) act on.
What are the repeated units that make up antibodies called?
Immunoglobulin domains (Ig domains) - these are the repeated protein units that make up many proteins including antibodies
Describe the structure of immunoglobulin domains in antibodies
The Ig domains form a Ig protein domain fold called a beta-barrel ~110 amino acids
- two anti-parallel b-pleated sheets are joined by disulphide bond
- made up of 3 loops which can vary their amino acid sequences without affecting stability
How many proteins make up an antibody molecule?
4 protein chains made up of repeating Ig domains
2 heavy chains made up of 4-5 domains
2 light chains made up of 2 domains
How are the proteins of an antibody molecule bonded together?
One heavy chain is bonded to a light chain via disulphide bond
Two heavy chains are bonded via disulphide bond
Therefore
L - H - H - L
Describe the structure of antibodies
Made up of protein chains made up of Ig domains
Forms a Y shaped molecule with two flexible arms with antigen binding sites located at the tip of these arms
What are the two regions of an antibody?
(Fab) Antigen binding region (consisting of light and heavy chains)
This region contains the antigen binding sites made from the N terminal domains of light and heavy chains
(Fc) Effector region - invariant and contains Fc receptors and complement component C1
What are the 5 classes of immunoglobulins (antibodies)?
IgM IgG IgD IgE IgA
What is IgM?
A class of immunoglobulins:
- default antibody made by all naive B cells
What are the two forms of IgM?
Membrane form and soluble form
What is the membrane form of IgM?
This is the monomer of the antibody which makes up the B-cell antigen receptor
What is the soluble form of IgM?
Pentamer with 10 antigen binding sites
These molecules attach to pathogens allowing complement component C1 to bind to its 5 Fc regions and cause opsonisation to occur
What is affinity?
When the sum of the attractive molecular forces at two surfaces exceed the repulsive forces
Higher the affinity, the fewer molecules it takes per unit volume to associate and to dissociate slowly
What is avidity?
The force of attraction occurring from multiple affinity contacts - way in which immune system reacts with pathogens instead of self antigens (difference in avidity
What is complementarity?
The idea that the antigen binding site of an antibody can form attractive forces to basically anything due to its diversity of amino acids making up the binding site
Where does variation of amino acids occur in antibodies?
3 discrete regions called complementarity determining regions (CDR)
These are the 3 loops that connect the strands in the 1st Ig domains of the heavy and light chains
What is the antigen binding region of an antibody made up of?
Each antibody contains 2 identical antigen binding sites
Each of these binding sites contain a heavy and light chain with the Ig domain forming 3 variable loops on each of these chains.
How do the CDR of antibodies vary?
The genes that code for the amino acids in the complementarity determining region of the antigen binding sites are segmented into variable, diversity, joining and constant regions (VDJC)
RAG1 and RAG2 (recombination activation genes) randomly select a single gene from a number of different genes in each segment and combine then removing every gene in between those that are selected.
In the CDR of the heavy chain, a gene from the D segment is joined to the J segment, then a gene from the V segment is joined to the D segment.
The process of joining genes is imprecise leading to loss of base pairs and hence more variation at the VDJ join.
VDJ join region codes for CDR3.
What is clonal selection?
The idea that a single B cell contains receptors for a single antigen.
Humans have many B cells at birth.
Explain the stages of affinity maturation in B cells
- A massive repertoire of naive B cells exist before birth each containing a unique B cell receptor
- B cells encounter antigens in lymph nodes - a small number of B cell clones with receptors that bind the antigen are produced
- Random somatic hypermutation of the Ig gene results in some clones with higher antigen receptor affinity
- The mature B cell becomes a plasma cell (high affinity B cell clone) which secretes soluble immunoglobulins
- Some B cells reside in the lymph nodes as long-term memory cells
