Lecture 23: Antibodies and gene rearrangement

Question 1

What are the features of adaptive immunity?

Answer 1

1. has memory - secondary response is stronger and faster than primary response
2. affinity of B cells towards antigen increase with time and persistence of antigen
3. 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

Question 2

What is a transposon?

Answer 2

A DNA sequence which can move its position in the genome

This gene was inserted into a receptor gene and transposase enzymes were extracted

Question 3

What is RAG?

Answer 3

Recombination activation genes are enzymes in human genome that allow movement rearrangement of genes

There are two types RAG1 and RAG2

Question 4

What are recognition sequences?

Answer 4

RS - are base pair sequences found at the end of genes which rearrange. Recognition sequences are where recombination activation genes (RAGs) act on.

Question 5

What are the repeated units that make up antibodies called?

Answer 5

Immunoglobulin domains (Ig domains) - these are the repeated protein units that make up many proteins including antibodies

Question 6

Describe the structure of immunoglobulin domains in antibodies

Answer 6

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

Question 7

How many proteins make up an antibody molecule?

Answer 7

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

Question 8

How are the proteins of an antibody molecule bonded together?

Answer 8

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

Question 9

Describe the structure of antibodies

Answer 9

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

Question 10

What are the two regions of an antibody?

Answer 10

(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

Question 11

What are the 5 classes of immunoglobulins (antibodies)?

Answer 11

IgM IgG IgD IgE IgA

Question 12

What is IgM?

Answer 12

A class of immunoglobulins:

• default antibody made by all naive B cells

Question 13

What are the two forms of IgM?

Answer 13

Membrane form and soluble form

Question 14

What is the membrane form of IgM?

Answer 14

This is the monomer of the antibody which makes up the B-cell antigen receptor

Question 15

What is the soluble form of IgM?

Answer 15

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

Question 16

What is affinity?

Answer 16

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

Question 17

What is avidity?

Answer 17

The force of attraction occurring from multiple affinity contacts - way in which immune system reacts with pathogens instead of self antigens (difference in avidity

Question 18

What is complementarity?

Answer 18

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

Question 19

Where does variation of amino acids occur in antibodies?

Answer 19

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

Question 20

What is the antigen binding region of an antibody made up of?

Answer 20

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.

Question 21

How do the CDR of antibodies vary?

Answer 21

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.

Question 22

What is clonal selection?

Answer 22

The idea that a single B cell contains receptors for a single antigen.

Humans have many B cells at birth.

Question 23

Explain the stages of affinity maturation in B cells

Answer 23

1. A massive repertoire of naive B cells exist before birth each containing a unique B cell receptor
2. B cells encounter antigens in lymph nodes - a small number of B cell clones with receptors that bind the antigen are produced
3. Random somatic hypermutation of the Ig gene results in some clones with higher antigen receptor affinity
4. The mature B cell becomes a plasma cell (high affinity B cell clone) which secretes soluble immunoglobulins
5. Some B cells reside in the lymph nodes as long-term memory cells